Cleanup codec tests

[easwars]

Existing codec tests in test/end2end_test.go should be moved to the codec directory and should be rewritten as e2e style tests:

• TestForceCodecName
• TestForceServerCodec
• TestEncodeDoesntPanic

Also, add a test where the client uses an unregistered codec on the server. Currently this results in proto codec being used. But this behavior will change soon, and having a test is a necessity at this point. See:
#6658