Add bounded test directory concurrency

[NickSdot]

Extracted from #22917.

Removes unnecessary directory wide serialisation from Curl, OpenSSL and open_basedir tests while keeping their resource use bounded.

MAX_CONCURRENCY sets a per directory worker limit. SCOPED_CONFLICTS allows tests within that directory to share a conflict key while still excluding outside users; existing CONFLICTS semantics are unchanged. Fixtures and ports are isolated where required.

Result

Values in seconds; measured on macOS.

mode	master	this PR	saved	speedup
-j10	94.007	19.757	74.250	4.76×

Reproduction:

sapi/cli/php run-tests.php -q -j10 ext/curl ext/openssl tests/security