selftests: gpio: gpio-sim: avoid forking test twice

Use '-o' within [] in order to avoid spawning two processes for test.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>