Fixed test filter
auto merge of #4169 : IdeaHat/servo/fix-mach-unit-test-filter, r=jdm

Minor change, added unit test filter to components so that ./mach test-unit [test-filter] works in line with documentation.

I'd personally like to also make filters on the components as well as the tests. This would change the interface (probably to ./mach test-unit [component-filter] [test-filter]), but change is NOT in this pull request.