os: use testenv.Command and os.Executable in tests

os: use testenv.Command and os.Executable in tests

On Unix platforms, testenv.Command sends SIGQUIT to stuck commands
before the test times out. For subprocesses that are written in Go,
that causes the runtime to dump running goroutines, and in other
languages it triggers similar behavior (such as a core dump).
If the subprocess is stuck due to a bug (such as #57999), that may
help to diagnose it.

For #57999.

Change-Id: I00f381b8052cbbb1a7eea90e7f102a3f68c842d1
Reviewed-on: https://go-review.googlesource.com/c/go/+/521817
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>