Make run-tests.php compatible with php 7.0-7.2

Make run-tests.php compatible with php 7.0-7.2

Motivation:
As an extension author, I want to speed up running tests in php <=7.3,
both locally and in CI (e.g. with valgrind).
This can be done by manually copying php 7.4's run-tests.php script
to replace the one generated by `phpize`

- list() doesn't work in php 7.0
- negative string offset doesn't work in php 7.2

If run-tests.php can be copied from php-src without any manual patches,
that would be the easiest.

Related to #2822 - I didn't see any discussion for/against compatibility
with older php versions