internal/testenv: in HasExec, try to actually exec on ios and wasm platforms

internal/testenv: in HasExec, try to actually exec on ios and wasm platforms

Some iOS environments may support exec. wasip1 and js do not, but
trying to exec on those platforms is inexpensive anyway and gives
better test coverage for the ios path.

Change-Id: I4baffb2ef5dc7d81e6a260f69033bfb229f13d92
Reviewed-on: https://go-review.googlesource.com/c/go/+/486275
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>