dev: ignore changes to _test.go files

Created by: keegancsmith

We don't need to recompile if a _test.go file is changed since they don't contribute to main. This change updates watchman to exclude triggering updates in that case. We only do this for watchman since the other file watchers do not make it as easy to express these sorts of rules. I also want to one day make watchman the only way to do this.