The test binaries accept -timeout=0 to mean no timeout,
but then the backup timer in cmd/go kills the test after 1 minute.
Make cmd/go understand this special case and change
behavior accordingly.

Fixes #14780.

Change-Id: I66bf517173a4ad21d53a5ee88d163f04b8929fb6
Reviewed-on: https://go-review.googlesource.com/81499
Run-TryBot: Russ Cox <rsc@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>