[libcxx patch] Modify testit to allow filtering tests by prefixes

[Albert Wong (王重傑)]

Hello!

This was a simple modification to testit which allows for filtering test
based on an environment variable. It was originally done to the android ndk
variant of this script (where it was very useful for reducing
compile/run/debug cycle times) so I thought I'd backport it incase anyone
wanted it.

Usage would be:

libcxx/test/language.support/support.dynamic/new.delete/new.delete.single$
TEST_PREFIX=new_replace ../../../../testit

-Albert
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