Use separate namespaces in the Python testdriver

MR !11611 (closed) uncovered a bug in the testsuite: we were defining a function noCapabilityOutputFilter in one file but using it in a different one.

This was not caught by a normal build because we load .T files using exec(src), so everything shares the same namespace. This is very non-modular and unreliable; we can have a similar crash in the future if the scanning of .T files uses a different order. The function os.walk (called in findTFiles) gives no guarantees on the ordering.