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

cmd/pack: 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: Ia1dd1bacfa2c9378401bbce304ebf7ee3f159352
Reviewed-on: https://go-review.googlesource.com/c/go/+/450707
Run-TryBot: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>