We are adding extra (non-phpt) test suites in [1] and [2]. In order to
avoid touching CI files too often (which are maintained in 8.1 and merged in
upper branches), we add a single entry point to call the extra tests. The entry
point can be updated in branches without synchronizing all the way from 8.1.
CI files still need to be touched to install dependencies of these tests,
but this should be manageable as these do not change often and are the same
in every branch.
Closes GH-19242.
[1] https://github.com/php/php-src/pull/16987
[2] https://github.com/php/php-src/pull/18939
