PEP8 line length fixes in testing_commands.py

...so that PyLint stops yelling at me
Adds a --component (or -c) argument to test-unit

Example usage:

`./mach test-unit -c style`
Last second bug catch
auto merge of #4229 : mttr/servo/mach_unit_test_select_component, r=Manishearth

Example usage:

`./mach test-unit -c style`