TestClientSupportedCompressors fails if other tests register compressors

[arjan-bal]

While adding a new compressor test in #7938, it was seen that registering a new compressor during one test causes TestClientSupportedCompressors to fail. TestClientSupportedCompressors verifies the list of compressors supported by clients. If a test compressor is registered in the same test file, it shows up in the list of supported compressors, failing the test, example failure logs.

There is a deprecated WithCompressor, dial option that can be used without registering a compressor. We should find a way to ensure that tests don't influence each other in such a way.

[arjan-bal]

@easwars I wanted to make the expected changes more concrete for other to pick this issue up. We could change TestClientSupportedCompressors to check if the expected codecs are a subset of the codecs got in the test. However, the problem seems to be that registering a codec during tests causes it to show up in the grpc-accept-encoding header, which we want to avoid. Let me know if this is correct.

One approach I could think of was thinking of the following approach:

1. Add a function named something like RegisterCompressorForTest(c Codec) func() in the internal package which returns a cleanup function that can be called to unregister/revert to the original codec after the test.
2. Make the codec package set the above function in an init() method.
3. Use the RegisterCompressorForTest in tests.

Does this sound good to you?