cmd/link: use testenv.Command instead of exec.Command in tests

cmd/link: use testenv.Command instead of exec.Command in tests

testenv.Command sets a default timeout based on the test's deadline
and sends SIGQUIT (where supported) in case of a hang.

Change-Id: I25a67aa5793c6fa977ff6d4dc59ca1533bf58d41
Reviewed-on: https://go-review.googlesource.com/c/go/+/450702
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Than McIntosh <thanm@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>