test: disable mTLS during tests (GoogleCloudPlatform#3822)

Disable mTLS during tests to increase the execution speed. The check whether
mTLS is available is otherwise executed for every new SpannerStub creation, and this
check is relatively slow, especially on macos. This has caused the macos tests to
time out regularly in the past week.

Author: olavloite

src/test/java/com/google/cloud/spanner/pgadapter/AbstractMockServerTest.java

@@ -23,6 +23,7 @@
 import com.google.cloud.NoCredentials;
 import com.google.cloud.ServiceOptions;
 import com.google.cloud.spanner.Dialect;
+import com.google.cloud.spanner.DisableDefaultMtlsProvider;
 import com.google.cloud.spanner.ErrorCode;
 import com.google.cloud.spanner.MockSpannerServiceImpl;
 import com.google.cloud.spanner.MockSpannerServiceImpl.StatementResult;
@@ -1282,6 +1283,8 @@ protected static void doStartMockSpannerAndPgAdapterServers(
       Consumer<TestOptionsMetadataBuilder> optionsConfigurator,
       OpenTelemetry openTelemetry)
       throws Exception {
+    DisableDefaultMtlsProvider.disableDefaultMtlsProvider();
+
     mockSpanner = mockSpannerService;
     mockSpanner.setAbortProbability(0.0D); // We don't want any unpredictable aborted transactions.
     mockSpanner.putStatementResult(