cmd/go: in TestScript, set GOTRACEBACK and use SIGQUIT to terminate hung subprocesses

cmd/go: in TestScript, set GOTRACEBACK and use SIGQUIT to terminate hung subprocesses

This should produce more useful outputs if a cmd/go invocation hangs
during a test.

(In some cases the outputs will be very verbose, but verbose is still
better than uninformative.)

For #36460
For #38768

Change-Id: Ibaf2d0fbf7387dfab1aad8981140ecb11901fc4b
Reviewed-on: https://go-review.googlesource.com/c/go/+/308809
Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
Reviewed-by: Jay Conrod <jayconrod@google.com>