The order of expected files should not be enforced.
Move worker.destroy() tests to end of instance lifecycle block.
Test worker.destroy().
Add before/afterEach for method test cases.
Expose readFiles utility as static property of LessWorker class.
Cover worker.start().
Cover worker.doneWrote().
Cover worker.inDir().
Share file-related variables throughout method test cases.

Also makes the filename an actual path.
Cover worker.writeOutput().
s/_getChildPath/resolveChildPath/g
Improve rebaseRootPath() pending tests, and add some for resolveChildPath().
Cover worker.resolveChildPath().
Share more variables in method test cases, with less ambiguous names.
Add external.css fixture.
Cover worker.rebaseRootPath().
Bump coverage thresholds to 90%.