Change run-tests.sh to use Symfony Process instead of proc_open

So wanted to re run a few times to verify consistently passes but seems to be failing now

Just rebased with no conflicts. No clue why it was failing earlier.

Guess one of those weird things. Re-ran again no issues.

larowlan → closed merge request !9565

Committed to 11.x and added a change record for the change to simpletest_script_command even though that's probably internal.

Automatically closed - issue fixed for 2 weeks with no activity.