tests(Spanner): Fix flakes in PooledSessionTests.

When using ForPartsOf NSubstitute calls the base methods, even on setup. Our base client SpannerClient implements all methods by throwing NotImplementedException. It seems there are cases where NSubstitute does not gracefully handle the exception.

Author: amanda-tarafa

apis/Google.Cloud.Spanner.Data/Google.Cloud.Spanner.Data.Tests/V1/PooledSessionTests.cs

@@ -140,6 +140,7 @@ await client.Received(1).ExecuteSqlAsync(
                     Assert.Equal(s_readWriteOptions, request.Transaction.Begin);
                 }),
                 Arg.Any<CallSettings>());
+            await client.DidNotReceive().BeginTransactionAsync(Arg.Any<BeginTransactionRequest>(), Arg.Any<CallSettings>());
         }
 
         [Fact]

apis/Google.Cloud.Spanner.Data/Google.Cloud.Spanner.Data.Tests/V1/SpannerClientHelpers.cs

@@ -58,7 +58,7 @@ internal static SpannerClient CreateMockClient(Logger logger)
             settings.Scheduler = fakeScheduler;
             settings.Clock = fakeScheduler.Clock;
             settings.Logger = logger;
-            var mock = Substitute.ForPartsOf<SpannerClient>();
+            var mock = Substitute.For<SpannerClient>();
             mock.Settings.Returns(settings);
             return mock;
         }