Fix tests

Author: sakthivelmanii

google-cloud-spanner/src/test/java/com/google/cloud/spanner/RetryOnDifferentGrpcChannelMockServerTest.java

@@ -284,6 +284,7 @@ public void testDenyListedChannelIsCleared() {
 
   @Test
   public void testSingleUseQuery_retriesOnNewChannel() {
+    assumeFalse(TestHelper.isMultiplexSessionDisabled());
     SpannerOptions.Builder builder = createSpannerOptionsBuilder();
     builder.setSessionPoolOption(
         SessionPoolOptions.newBuilder().setUseMultiplexedSession(true).build());

google-cloud-spanner/src/test/java/com/google/cloud/spanner/TestHelper.java

@@ -1,3 +1,18 @@
+/*
+ * Copyright 2025 Google LLC
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
 package com.google.cloud.spanner;
 
 class TestHelper {