Fix failing test.

This test was failing, as `foo` was being set in the `beforeEach` (with
`force: true`), so it always had a value.
Allow paths to be mocked for easier testing.

This allows consuming libraries to mock the `path` and `homePath` during
their own test suites.
Merge pull request #1 from rjackson/allow-paths-to-be-set

Allow `path` & `homePath` to be mocked for easier testing.