My previous fix for the test runner fixed the case where the specific
tests to be run were appearing in the test_labels argument, preventing
all tests from being run. However, it broke the case where test_labels
was empty but specific tests were provided in argv.

This change fixes both cases by appending any non-option values from
test_labels that don't exist in argv already and then checking the
resulting argv for specific tests to run. If none are found, we default
to running all tests from the configured test packages.

Tested running all tests and specific tests from Djblets, Review Board,

and RBCommons, with and without using --, ensuring all the cases I've

ran into were fixed.