Allow running just one test with tox

Now, "tox -e py26 -- test.unit.proxy.test_server" runs just that one
test file.

"tox -e py26" still runs all the tests with py26, just like before.

Change-Id: I40db12dd5e7cc8f9388e29b30447f70d3bfc4b28
Merge "Allow running just one test with tox"