run-tests.php: Add a flag similar to --asan that skips tests when run under Valgrind

[Girgias]

Description

I just ran 11k tests under Valgrind to try and find issues with one of my big PRs, and any test excepting an OOM failure will either time out or use all the memory before being killed.

It would be great if those could be skipped and not accidentally triggered.

Some relevant tests:

• ext/standard/tests/gh14643_longname.phpt
• Zend/tests/bug79836_4.phpt
• tests/basic/bug67198.phpt (maybe)
• tests/basic/enable_post_data_reading_05.phpt (maybe)

Other tests are those opening PHP procs and timing out:

• ext/standard/tests/file/proc_open01.phpt
• ext/standard/tests/http/gh11274.phpt
• ext/standard/tests/streams/bug64433.phpt
• ext/standard/tests/url/get_headers_error_003.phpt

And some tests seem to be bogus with Valgrind or an issue that we cannot fix:

• ext/standard/tests/mail/mail_basic2.phpt
• ext/standard/tests/mail/mail_variation2.phpt