TST: Update baseline image for density test

The old image had some wrong dimensions (not the current ggplot default) and
so the test were failing.
TST: Update baseline images for matplotlib theme

Although the tests were not failing, the old baseline images had a different
and wrong background.
TST: Update baseline images for geom_text

The old baseline image had serif text, but that is not the default (sans serif).
TST: Add missing baseline images

These images were missing due to a failing testcase which prevented
the images to be generated and added.
TST: Visual test script: mark non existing current images as failure

The old code would assume that no produced images (due to a exception
in the code would be a "passed" test. Now, a non-produced images marks
the test as not passed in the html output of the visual test.
TST: Split test_basic in smaller tests