[iOS] Fix ZipPackage_CreateWithFileAccessWrite test (#72850)

On iOS, this test was failing due to not being able to write files in the app root directory.  The fix is to write a file in the TestDirectory, which is a subdirectory under the app root and is writeable.

Fixes #72837

src/libraries/System.IO.Packaging/tests/Tests.cs

@@ -3698,7 +3698,7 @@ public void CreateWithFileAccessWrite()
         [SkipOnTargetFramework(TargetFrameworkMonikers.NetFramework, "Desktop doesn't support Package.Open with FileAccess.Write")]
         public void ZipPackage_CreateWithFileAccessWrite()
         {
-            string packageName = "test.zip";
+            string packageName = Path.Combine(TestDirectory, "test.zip");
 
             using (Package package = Package.Open(packageName, FileMode.Create, FileAccess.Write))
             {