Use `__DIR__` magic constant instead of `dirname(__FILE__)`

[ayeshrajans]

Now with WordPress requiring PHP 5.6 the minimum, we can replace dirname(__FILE__) calls with the PHP 5.3-introuced __DIR__ constant.

This ticket does not intend to change such calls in third party libraries, even on the ones that we are the de-facto maintainers due to the package age.

[SergeyBiryukov]

Thanks for the patch!

class-requests.php is a part of the ​Requests external library, any changes should be submitted upstream.

The change in wp-tests-config-sample.php was previously reverted in [47201] to avoid breaking unit tests created with WP-CLI scaffold command, see comment:15:ticket:48082 and #49377 for details.

[desrosj]

Going to close this one out in favor of submitting the changes upstream to the Requests library.

If in the future updating wp-tests-config-sample.php becomes possible without the backwards compatibility problems, this can be reopened or a new ticket can be created.