test-lib: allow selecting tests by substring/glob with --run
Many of our test scripts have several "setup" tests. It's a lot easier
to say
./t0050-filesystem.sh --run=setup,9
in order to run all the setup tests as well as test #9, than it is to
track down what all the setup tests are and enter all their numbers in
the list. Also, I often find myself wanting to run just one or a couple
tests from the test file, but I don't know the numbering of any of the
tests -- to get it I either have to first run the whole test file (or
start counting by hand or figure out some other clever but non-obvious
tricks). It's really convenient to be able to just look at the test
description(s) and then run