cmd/go: don't be clever about mtime precision in test
This doesn't happen enough in the tests to be worth debugging.
Empirically, I expect this to add 5 seconds to the overall 'go test -short cmd/go'
on systems with precise file systems, and nothing on systems without them
(like my Mac).
Fixes #12205.
Change-Id: I0a17cb37bdedcfc0f921c5ee658737f1698c153b
Reviewed-on: https://go-review.googlesource.com/17953 Reviewed-by: Ian Lance Taylor <iant@golang.org>