os/exec: skip remount in TestFindExecutableVsNoexec on EROFS

os/exec: skip remount in TestFindExecutableVsNoexec on EROFS

To allow using testenv.SyscallIsNotSupported, rewrite the test to use
the exported API only. Given that path is an absolute path,
exec.LookPath is equivalent to exec.findExecutable on linux.

Fixes #59087

Change-Id: Ia01b84d4e9d5a65a88dd995f9e3c8a81c4ccd19f
Reviewed-on: https://go-review.googlesource.com/c/go/+/481620
Reviewed-by: Bryan Mills <bcmills@google.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Bypass: Bryan Mills <bcmills@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>