os: use testenv.Command instead of exec.Command in tests

os: 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: I32ea9ca11c30d8af3d5490f2db1674314962cc80
Reviewed-on: https://go-review.googlesource.com/c/go/+/451195
Reviewed-by: Bryan Mills <bcmills@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Joedian Reid <joedian@golang.org>
Auto-Submit: Bryan Mills <bcmills@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>