chore: use client test fixture to speedup tests (#81)

This PR introduces a client test fixture. The previous tests
instantiated the client in test class constructors. That is, we used
this pattern:

```csharp
public class MyTests
{
	public MyTests()
	{
		// Instantiate some things
	}

	public void Test_Something()
	{
		// tests
	}

	public void Test_Otherthing()
	{
		// tests
	}
}
```

Data instantiated in the constructor will be setup and torn down
for *each* test method, not just for the class as a whole. This is not
the same thing `setUp` from jUnit.

In Fixtures.cs we create a client fixture and register it to be
available across all test classes. Other tests modified appropriately.

MomentoIntegrationTest/Fixtures.cs

@@ -0,0 +1,46 @@
+namespace MomentoIntegrationTest
+{
+    /// <summary>
+    /// A cache client fixture.
+    /// Use this when not testing client-building edge cases:
+    /// re-using the client drops overall integration test time down ~5X.
+    /// </summary>
+    public class SimpleCacheClientFixture : IDisposable
+    {
+        public SimpleCacheClient Client { get; private set; }
+        public string AuthToken { get; private set; }
+
+        public const string CacheName = "client-sdk-csharp";
+        public const uint DefaultTtlSeconds = 10;
+
+        public SimpleCacheClientFixture()
+        {
+            AuthToken = Environment.GetEnvironmentVariable("TEST_AUTH_TOKEN") ??
+                throw new NullReferenceException("TEST_AUTH_TOKEN environment variable must be set.");
+            Client = new(AuthToken, defaultTtlSeconds: DefaultTtlSeconds);
+
+            try
+            {
+                Client.CreateCache(CacheName);
+            }
+            catch (AlreadyExistsException)
+            {
+            }
+        }
+
+        public void Dispose()
+        {
+            Client.DeleteCache(CacheName);
+            Client.Dispose();
+        }
+    }
+
+    /// <summary>
+    /// Register the fixture in xUnit.
+    /// </summary>
+    [CollectionDefinition("SimpleCacheClient")]
+    public class SimpleCacheClientCollection : ICollectionFixture<SimpleCacheClientFixture>
+    {
+
+    }
+}

MomentoIntegrationTest/SimpleCacheControlTest.cs

@@ -1,22 +1,24 @@
-using System;
-using System.Linq;
-using Xunit;
-using MomentoSdk;
-using MomentoSdk.Exceptions;
-using MomentoSdk.Responses;
+using System.Linq;
 using System.Collections.Generic;
 
 namespace MomentoIntegrationTest
 {
+    [Collection("SimpleCacheClient")]
     public class SimpleCacheControlTest
     {
-        private string authKey = Environment.GetEnvironmentVariable("TEST_AUTH_TOKEN") ??
-            throw new NullReferenceException("TEST_AUTH_TOKEN environment variable must be set.");
+        private SimpleCacheClient client;
+        private string authToken;
+
+        public SimpleCacheControlTest(SimpleCacheClientFixture fixture)
+        {
+            client = fixture.Client;
+            authToken = fixture.AuthToken;
+        }
 
         [Fact]
         public void SimpleCacheClientConstructor_BadRequestTimeout_ThrowsException()
         {
-            Assert.Throws<InvalidArgumentException>(() => new SimpleCacheClient(authKey, defaultTtlSeconds: 10, dataClientOperationTimeoutMilliseconds: 0));
+            Assert.Throws<InvalidArgumentException>(() => new SimpleCacheClient(authToken, defaultTtlSeconds: 10, dataClientOperationTimeoutMilliseconds: 0));
         }
 
         [Fact]
@@ -34,21 +36,18 @@ public void SimpleCacheClientConstructor_NullJWT_InvalidJwtException()
         [Fact]
         public void DeleteCache_NullCache_ArgumentNullException()
         {
-            SimpleCacheClient client = new SimpleCacheClient(authKey, defaultTtlSeconds: 10);
             Assert.Throws<ArgumentNullException>(() => client.DeleteCache(null!));
         }
 
         [Fact]
         public void DeleteCache_CacheDoesntExist_NotFoundException()
         {
-            SimpleCacheClient client = new SimpleCacheClient(authKey, defaultTtlSeconds: 10);
-            Assert.Throws<NotFoundException>(() => client.DeleteCache("non existant cache"));
+            Assert.Throws<NotFoundException>(() => client.DeleteCache("non-existent cache"));
         }
 
         [Fact]
         public void CreateCache_NullCache_ArgumentNullException()
         {
-            SimpleCacheClient client = new SimpleCacheClient(authKey, defaultTtlSeconds: 10);
             Assert.Throws<ArgumentNullException>(() => client.CreateCache(null!));
         }
 
@@ -57,7 +56,6 @@ public void CreateCache_NullCache_ArgumentNullException()
         public void ListCaches_OneCache_HappyPath()
         {
             // Create cache
-            SimpleCacheClient client = new SimpleCacheClient(authKey, defaultTtlSeconds: 10);
             string cacheName = Guid.NewGuid().ToString();
             client.CreateCache(cacheName);
 
@@ -77,7 +75,6 @@ public void ListCaches_OneCache_HappyPath()
         public void ListCaches_Iteration_HappyPath()
         {
             // Create caches
-            SimpleCacheClient client = new SimpleCacheClient(authKey, defaultTtlSeconds: 10);
             List<String> cacheNames = new List<String>();
 
             // TODO: increase limit after pagination is enabled
@@ -119,7 +116,6 @@ public void ListCaches_Iteration_HappyPath()
         public void ListCaches_BadNextToken_NoException()
         {
             // A bad next token does not throw an exception
-            SimpleCacheClient client = new SimpleCacheClient(authKey, defaultTtlSeconds: 10);
             client.ListCaches(nextPageToken: "hello world");
         }
     }