Also, this commit adds a test for ensuring that TestMain(t *testing.T) is a normal test.

Fixes #22388

Change-Id: Iffcb1db5cdcf34b9c822fcdb58f8926535415177
Reviewed-on: https://go-review.googlesource.com/72591
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>