cmd/cgo/internal/testplugin: simplify TestForkExec and log stderr output

cmd/cgo/internal/testplugin: simplify TestForkExec and log stderr output

This test used to run with a separate goroutine for timeout behavior,
presumably because it was difficult to set an appropriate timeout.

Now that the test is in cmd instead of misc, we can use
internal/testenv.Command, which makes adding the test timeout much
simpler and eliminates the need for the explicit goroutine.

For #61846.

Change-Id: I68ea09fcf2aa394bed1e900cf30ef7d143fa249f
Reviewed-on: https://go-review.googlesource.com/c/go/+/517095
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Bryan Mills <bcmills@google.com>
Auto-Submit: Bryan Mills <bcmills@google.com>