Remove debugging cruft from run-tests.py

No need to write Valgrind's xml output to disk.
Provide a way to only run a subset of all tests

By setting the environment variable T to a set of test specs, it is
possible to limit which tests are run by 'make test'.

Example: run all tests in test-adb:

    $ T=test-adb make test

Example: run a single test in test-adb and all tests in
test-ring-buffer under Valgrind:

    $ VALGRIND=1 T='test-adb:adb_callback test-ring-buffer' make test