Skip external HTTP test for recommended PHP and MySQL versions on 503 errors

[SergeyBiryukov]

Background: #40533, #44613.

• [33937] / [33946] introduced test_readme(), which performs HTTP requests to php.net and dev.mysql.com to verify that the recommended PHP and MySQL versions in readme.html are up-to-date.
• [40519] made the test fail if php.net or dev.mysql.com are unreachable, in order to inform us of potential URL changes.

This test currently fails with a 503 Service Temporarily Unavailable error from php.net and the following response body, which appears to be some kind of DoS protection:

protection exception WP1X

While I agree that the test should not be skipped if the resources are permanently unavailable, failing for temporary issues outside of our control is not ideal.

I think the test should skip, not fail, for 503 errors.

On a related note, https://secure.php.net/ now performs a 301 redirect to https://www.php.net/, so the URL in the test could be updated.

[SergeyBiryukov]

In 46998:

Tests: Skip external HTTP test for recommended PHP and MySQL versions on 503 errors.

Fixes #49049.