Added detection of the mint-themes-gtk3 repository in parent-directory.

It will be automatically bind mounted if it's detected in the parent directory,
which allows testing both themes together.

Added a dummy gtk-3.0/ directory to allow the new bind mounting script to
succeed.
Merge pull request #38 from wrouesnel/improved-test-script

Added detection of the mint-themes-gtk3 repository in parent-directory.