remove redudnant cast to IReadOnlyList and add a not null ckeck in AssemblyEnumeratorTests (#5402)

Author: SimonCropp

test/UnitTests/MSTestAdapter.UnitTests/Discovery/AssemblyEnumeratorTests.cs

@@ -94,7 +94,7 @@ public void GetTypesShouldReturnSetOfDefinedTypes()
         // Setup mocks
         mockAssembly.Setup(a => a.GetTypes()).Returns(expectedTypes);
 
-        IReadOnlyList<Type> types = AssemblyEnumerator.GetTypes(mockAssembly.Object, string.Empty, _warnings);
+        Type[] types = AssemblyEnumerator.GetTypes(mockAssembly.Object, string.Empty, _warnings);
         Verify(expectedTypes.SequenceEqual(types));
     }
 
@@ -116,7 +116,7 @@ public void GetTypesShouldReturnReflectionTypeLoadExceptionTypesOnException()
         // Setup mocks
         mockAssembly.Setup(a => a.GetTypes()).Throws(new ReflectionTypeLoadException(reflectedTypes, null));
 
-        IReadOnlyList<Type> types = AssemblyEnumerator.GetTypes(mockAssembly.Object, string.Empty, _warnings);
+        Type[] types = AssemblyEnumerator.GetTypes(mockAssembly.Object, string.Empty, _warnings);
 
         Verify(types is not null);
         Verify(reflectedTypes.Equals(types));
@@ -131,7 +131,10 @@ public void GetTypesShouldLogWarningsWhenReflectionFailsWithLoaderExceptions()
         mockAssembly.Setup(a => a.GetTypes()).Throws(new ReflectionTypeLoadException(null, exceptions));
         mockAssembly.Setup(a => a.GetTypes()).Throws(new ReflectionTypeLoadException(null, exceptions));
 
-        IReadOnlyList<Type> types = AssemblyEnumerator.GetTypes(mockAssembly.Object, "DummyAssembly", _warnings);
+        Type[] types = AssemblyEnumerator.GetTypes(mockAssembly.Object, "DummyAssembly", _warnings);
+
+        // Depending on the TFM, .NET either gives us null or empty array.
+        Verify(types is null || types.Length == 0);
 
         Verify(_warnings.Count == 1);
         Verify(_warnings.ToList().Contains(