Several test cases do not run on Linux

[Lukasa]

grpc-swift uses test discovery on Linux to find the CI tests. This mostly works well, but unlike on Darwin (where tests are discovered by querying the Objective-C runtime), on Linux the tests are discovered only by source code analysis. This analysis does not work with subclasses. As a result, any testing strategy that relies on subclasses discovering tests declared in parent classes fails to run on Linux. At a bare minimum, the following do not run on Linux:

• FunctionalTestsInsecureTransport subclasses FunctionalTestsAnonymousClient and FunctionalTestsMutualAuthentication do not run.
• GRPCInsecureInteroperabilityTests subclass GRPCSecureInteroperabilityTests does not run.
• The ServerThrowingTests subclasses ServerDelayedThrowingTests, ClientThrowingWhenServerReturningErrorTests, ServerErrorTransformingTests do not run.

All of these need to be rewritten to expose an entry point for the test in all subclasses.