cmd/go/internal/test: pass default timeout to test programs if not given from command line

Make 'go test' command to pass the default timeout (10m) to test programs if the value is not given from command line.

Fixes #28147

Change-Id: I7856e452224a51a92da03bab8e3a0f9d7c41d32a
GitHub-Last-Rev: 66f9a6f90e9ffe7c58d5c1fe32af84e16ea74ab8
GitHub-Pull-Request: golang/go#30545
Reviewed-on: https://go-review.googlesource.com/c/go/+/164963


Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Bryan C. Mills <bcmills@google.com>