context: Cleanup ContextTest, use final, remove throws, use assertNull (#6941)
context: Cleanup ContextTest, use final, remove throws, use assertNull (#6941)

Signed-off-by: Bogdan Drutu <3f8e17047c2434e5c82a61403e56b9ef3e226d99@gmail.com>
stanley-cheung/grpc-java,stanley-cheung/grpc-java,ejona86/grpc-java,grpc/grpc-java,stanley-cheung/grpc-java,grpc/grpc-java,dapengzhang0/grpc-java,ejona86/grpc-java,dapengzhang0/grpc-java,grpc/grpc-java,dapengzhang0/grpc-java,stanley-cheung/grpc-java,dapengzhang0/grpc-java,ejona86/grpc-java,ejona86/grpc-java,grpc/grpc-java