fix(java): exclude tests that use Mockito from native image testing

[mpeddada1]

Mockito dynamically loads classes at run-time and is currently incompatible with native image testing. Therefore, tests that use Mockito are resulting in the following error when they are executed with native image testing:

DdlClientTest:

  JUnit Vintage:DdlClientTest:testExecuteDdl
    MethodSource [className = 'com.google.cloud.spanner.connection.DdlClientTest', methodName = 'testExecuteDdl', methodParameterTypes = '']
    => com.oracle.svm.core.jdk.UnsupportedFeatureError: Proxy class defined by interfaces [interface org.mockito.plugins.PluginSwitch] not found. Generating proxy classes at runtime is not supported. Proxy classes need to be defined at image build time by specifying the list of interfaces that they implement. To define proxy classes use -H:DynamicProxyConfigurationFiles=<comma-separated-config-files> and -H:DynamicProxyConfigurationResources=<comma-separated-config-resources> options.
       com.oracle.svm.core.util.VMError.unsupportedFeature(VMError.java:87)
       com.oracle.svm.reflect.proxy.DynamicProxySupport.getProxyClass(DynamicProxySupport.java:146)
       java.lang.reflect.Proxy.getProxyConstructor(Proxy.java:66)
       java.lang.reflect.Proxy.newProxyInstance(Proxy.java:1006)
       org.mockito.internal.configuration.plugins.PluginLoader.loadPlugin(PluginLoader.java:81)
       org.mockito.internal.configuration.plugins.PluginLoader.loadPlugin(PluginLoader.java:54)
       org.mockito.internal.configuration.plugins.PluginRegistry.<init>(PluginRegistry.java:21)
       org.mockito.internal.configuration.plugins.Plugins.<clinit>(Plugins.java:22)
       org.mockito.internal.MockitoCore.<clinit>(MockitoCore.java:77)
       org.mockito.Mockito.<clinit>(Mockito.java:1614)
       [...]

SessionClientTest:

JUnit Vintage:SessionClientTest:[NumChannels = 8]:createAndCloseSession[NumChannels = 8]
    MethodSource [className = 'com.google.cloud.spanner.SessionClientTest', methodName = 'createAndCloseSession', methodParameterTypes = '']
    => java.lang.NoClassDefFoundError: Could not initialize class org.mockito.internal.configuration.plugins.Plugins
       org.mockito.internal.configuration.GlobalConfiguration.tryGetPluginAnnotationEngine(GlobalConfiguration.java:50)
       org.mockito.MockitoAnnotations.openMocks(MockitoAnnotations.java:81)
       org.mockito.MockitoAnnotations.initMocks(MockitoAnnotations.java:100)
       com.google.cloud.spanner.SessionClientTest.setUp(SessionClientTest.java:90)
       java.lang.reflect.Method.invoke(Method.java:566)
       org.junit.runners.model.FrameworkMethod$1.runReflectiveCall(FrameworkMethod.java:59)
       org.junit.internal.runners.model.ReflectiveCallable.run(ReflectiveCallable.java:12)
       org.junit.runners.model.FrameworkMethod.invokeExplosively(FrameworkMethod.java:56)
       org.junit.internal.runners.statements.RunBefores.invokeMethod(RunBefores.java:33)
       org.junit.internal.runners.statements.RunBefores.evaluate(RunBefores.java:24)
       [...]

Some background on how native image tests are opted in: The logic to opt libraries into native image testing lives in java-shared-config (see https://github.com/googleapis/java-shared-config/blob/1015b908c5aaa6f0b6c77759d86557ca26504868/pom.xml#L811) Currently tests with the IT* and *ClientTest pattern are opted in. The reason for this is to include integration tests in all libraries and unit tests in only generated libraries for native image testing (see this logic for the naming pattern the Gapic generator uses to generate unit tests).

This PR excludes SessionClientTest and DdlClientTest from native image testing by renaming them to SessionClientTests and DdlClientTests.

cc @ansh0l @meltsufin

[mpeddada1]

Hi @ansh0l could we get a review on this PR?

[ansh0l]

LGTM. @mpeddada1 Please wait on merging this one, I think we should batch all PRs for native image testing to have a single release. We're planning a release for cross region backups in next few days.

[ansh0l]

@mpeddada1 : I've added a do not merge label - feel free to remove and merge with other PRs when they are ready.

[ansh0l]

Closing since native image changes have been released with #1878