grpctest: minor improvements to the test logger implementation

[easwars]

I'd like to suggest the following minor improvements to the test logger implementation:

• Unexport the test logger implementation which is currently a package global here:

  grpc-go/internal/grpctest/tlogger.go

  Line 38 in 6fa393c

  var TLogger *tLogger

  • This would also require moving some methods on this type like ExpectError to be functions in the package
• Make the above package global accessed atomically from tests. We have had test failures because of data races in the past because of this.
• Change receiver names in method of the tLogger type to be tl instead of g