Use generic images for tests

The following commits in the taurus-docker project revert to using image names that are not binding-specific (but instead include both pyside and pyqt5 in all images):

• taurus-docker@5fe6dc83
• taurus-docker@d4da5023

Adapt our CI to that change.

Also make sure that the pyside2 tests are done using QT_API=pyside2