test: rename race.go to race_test.go

[easwars]

race.go has the following build constraints:

//go:build race
// +build race

and sets the package global variable raceMode to true.

The way things are currently, I get a build failure when I try to run all tests under grpc with --config==gotsan:

easwars@minetop: google3$ blaze test third_party/golang/grpc/... --config=gotsan
...
third_party/golang/grpc/test/race.go:24:2: undefined: raceMode

I don't quite understand why I see this failure, because if I do blaze test third_party/golang/grpc/test:test_test --config=gotsan, that works.

What I do understand though is that the we end up having two BUILD targets for the test package, test and test_test. The former contains files in this package which don't have an _test.go suffix. And renaming race.go to race_test.go puts in the test_test package and things work fine with this approach.

Also, race.go sets the raceMode variable which is defined in end2end_test.go. So, it makes sense for the former to be part of the test_test package.

RELEASE NOTES: none