resourceloader: Improve coverage of SkinModule

resourceloader: Improve coverage of SkinModule

* Ignore getLogoData() which is a one-line proxy to ::getLogo,
  that can't really be tested because static methods can't be stubbed.
  It exists so that this method can be stubbed instead. The actual
  method is already covered.

* Simplify @covers for getStyles() to allow indirect coverage within
  the class because it's intended as a higher-level integration tests.
  The other tests in the suite still cover a specific method only.

Change-Id: I1445a016c1f12a6d8ceaaf745023a28cf20e5371