Keepalive test improvements. (#2992)
Keepalive test improvements. (#2992)

* Added a test for keepalive dormancy.
* Added comments to all tests, explicitly stating their intention.
* Used the exported APIs wherever possible instead of relying on
  internal state.
* Specifically checked for GoAway reason, instead of just relying on the
  receipt of a goaway.
* Improved error messages printed upon test failures.
* Made the tests more consistent with each other in terms of variables
  names, whitespace usage etc.

Once this PR is approved and merged, we can move all these tests to a
separate file.
stanley-cheung/grpc-go,grpc/grpc-go,menghanl/grpc-go,dfawley/grpc-go,grpc/grpc-go,dfawley/grpc-go,stanley-cheung/grpc-go,menghanl/grpc-go