Support hierarchical test resource data, such that you can have
`tests/resources/foo/bar` and move the `bar` directory in as
a fixture.

Calling `cl_fixture_sandbox` on a path that is not directly beneath
the test resources directory succeeds, placing that directory into
the test fixture.  (For example, `cl_fixture_sandbox("foo/bar")`
will sandbox the `foo/bar` directory as `bar`).

Add support for cleaning up directories created this way, by only
cleaning up the basename (in this example, `bar`) from the fixture
directory.