Add TryAdd and Clear regression tests (dotnet/corefx#32407)

* Add TryAdd and Clear regression tests

* Add Run Condition on Clear()

* Address PR Feedback

* Address PR Feedback dotnet/corefx#2

*  Address PR Feedback dotnet/corefx#3

* Remove Extra Line

* Add MoveNext Result Asserts


Commit migrated from dotnet/corefx@d8a0778

src/libraries/System.Collections/tests/Generic/Dictionary/Dictionary.Tests.cs

@@ -126,6 +126,20 @@ public void IDictionary_NonGeneric_Contains_KeyOfWrongType(int count)
             }
         }
 
+        [Fact]
+        public void Clear_OnEmptyCollection_DoesNotInvalidateEnumerator()
+        {
+            if (ModifyEnumeratorAllowed.HasFlag(ModifyOperation.Clear))
+            {
+                IDictionary dictionary = new Dictionary<string, string>();
+                IEnumerator valuesEnum = dictionary.GetEnumerator();
+
+                dictionary.Clear();
+                Assert.Empty(dictionary);
+                Assert.False(valuesEnum.MoveNext());
+            }
+        }
+
         #endregion
 
         #region ICollection tests
@@ -250,6 +264,18 @@ public void Remove_NonExistentEntries_DoesNotPreventEnumeration()
             }
         }
 
+        [Fact]
+        public void TryAdd_ItemAlreadyExists_DoesNotInvalidateEnumerator()
+        {
+            var dictionary = new Dictionary<string, string>();
+            dictionary.Add("a", "b");
+
+            IEnumerator valuesEnum = dictionary.GetEnumerator();
+            Assert.False(dictionary.TryAdd("a", "c"));
+
+            Assert.True(valuesEnum.MoveNext());
+        }
+
         [Theory]
         [MemberData(nameof(CopyConstructorInt32Data))]
         public void CopyConstructorInt32(int size, Func<int, int> keyValueSelector, Func<IDictionary<int, int>, IDictionary<int, int>> dictionarySelector)