cmd/go: convert TestTestRegexps to the script framework

It's hard to convert this one exactly because I don't think
we can guarantee that the grep command exists to filter stdout,
so I've tried to replicate the intent of the test.

Part of converting all tests to script framework to improve
test parallelism.

Updates #36320
Updates #17751

Change-Id: Ib593799ef7634ce12efb3ff357eb34475e2ea321
Reviewed-on: https://go-review.googlesource.com/c/go/+/213130


Run-TryBot: Michael Matloob <matloob@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>