cmd/cover: use testenv.Command instead of exec.Command

cmd/cover: use testenv.Command instead of exec.Command

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

Change-Id: Ic19f8b020f6d410942bb2ece8a3b71607ee6488a
Reviewed-on: https://go-review.googlesource.com/c/go/+/450695
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>