grpc: do not apply default service config upon receipt of invalid service config

[easwars]

In #5238, we made a change to apply the default service config when grpc receives invalid service config from the name resolver (and a default service config exists).

But this does not sit well with 4.2:

If the client has not previously selected a valid service config or no service config,
for example a new client, then it treats the resolution attempt as having failed
(e.g., for a polling-based resolver, it should retry the query after appropriate backoff).
In other words, the channel will remain in state TRANSIENT_FAILURE until the
resolver returns a valid response.

This PR does the following:

• undoes some of the changes from grpc: handle invalid service configs by applying the default, if applicable #5238, by not applying the default service config
• Renames test/resolver_test.go to test/config_selector_test.go (since it contained only config selector related tests)
• Adds a new test/resolver_test.go which contains tests for different error handling scenarios upon receipt of resolver update. This pulls in a couple of tests from resolver_conn_wrapper_test.go, which is now deleted.

RELEASE NOTES: none

[dfawley]

Nice, thanks for adding the new test cases. Did you check in / to see if similar tests already existed there? I would not be surprised if there were similar ones, but also not surprised if there were not. :)

[dfawley]

Can you move this with the proper git magic to make it show up that way, please?

[easwars]

Done.

Earlier, I think I had done a git mv test/resolver_test.go test/config_selector_test.go. But then, added the new tests to a new file test/resolver_test.go. But git knew it as a old file. Hence the confusion. Thanks.

[easwars]

So, I had to put the tests from resolver_conn_wrapper_test.go and the new tests into resolver_update_test.go.

[easwars]

Nice, thanks for adding the new test cases. Did you check in / to see if similar tests already existed there? I would not be surprised if there were similar ones, but also not surprised if there were not. :)

I had done some basic grepping, and the only tests were the ones in the now deleted resolver_conn_wrapper_test.go. They were incomplete.