Fix batch track test command

[BNAndras]

See http://forum.exercism.org/t/track-specific-testing-documentation/12829/6 for context. exercism test currently doesn't work for Batch Script exercises in Command Prompt or PowerShell. I locally built testercism with the proposed cmd /c {{test_files}} command, and it worked in my limited testing within both shells.

[ErikSchierboom]

Note: this isn't automatically deployed. That's a manual task.