The test runner tries to respect the test_packages variable defined on

the test runner, to allow only a subset of tests to run by default.

However, this was broken, as we trusted the test_labels variable

provided to us, but this could easily support options masquerading as

test labels.
We now filter those test labels, removing anything that looks like an

option or is already specified in the nose arguments. Since those nose

arguments are built based on command line arguments, this should safely

remove everything but the test labels we want to run, and fall back on

the configured test_packages if empty.

Tested with RBCommons, which separates out the rbcommons tests from

the reviewboard tests using test_packages. When passing command

line arguments (like -x), test_packages was being ignored, but

now it's working fine and correctly using -x.
Tested standard test runs of Djblets and Review Board.