Fix breakage of `./mach test`

When running commands through Registrar.dispatch, mach does not behave
in the same way it would as if it were running through the command line.
Defaults normally provided through a combination of @CommandArgument and
argparse magic are ignored.

I have some ideas as to how to fix this, but until then, this will allow
`./mach test` to run through test-unit properly.
auto merge of #4269 : mttr/servo/mach_test_fix, r=mbrubeck

When running commands through Registrar.dispatch, mach does not behave
in the same way it would as if it were running through the command line.
Defaults normally provided through a combination of @CommandArgument and
argparse magic are ignored.

I have some ideas as to how to fix this, but until then, this will allow
`./mach test` to run through test-unit properly.