fix(test_image_content): Improve /usr symlink and conflict tests.

Now uses the package database instead of filesystem so the check works
even if /bin and friends are symlinks to /usr. Also disable the
whitelist and check that the expected symlinks are correct if the
symlink-usr USE flag is enabled.
Merge pull request #145 from marineam/symlink-usr

fix(test_image_content): Improve /usr symlink and conflict tests.