DRY up running tests

Moved duplication from `runFile` and `runTest` into `runTests`. Also
renamed `runTest` to `runSingle`.
Don't open the browser if `name` is undefined
Remove unused properties from grammars

Forgot to remove these.
Simplify NilGrammar

No need to define an `undefined` property. It’ll be `undefined` anyway.
Merge branch 'refactor'