Fix supposedly NTFS-related "races" for the `tmp` directory

This addresses #366 by using a different directory in the system's temporary directory for every test. It can still occur that the temp dirs are not correctly deleted after the test finishes but it causes no follow-up tests to fail anymore. I'd say that the occasional stray temp dir when running tests is an acceptable drawback.

Introduced a constant `Nanoc::TestHelpers::LIB_DIR` since some tests using `YARD.parse` relied on the relative position of the temp dir to the lib dir.