cmd/go: bail out from script tests earlier when a timeout occurs

(I needed this to debug an accidental infinite-recursion I introduced
while revising CL 293689.)

Fixes #38768

Change-Id: I306122f15b5bbd2fc5e836b32fd4dd5992ea891e
Reviewed-on: https://go-review.googlesource.com/c/go/+/302052


Trust: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>