fix tests

[k-dominik]

with recent dependency updates and merged branches tests started to fail

(my bad, didn't merge master before)

reasons:

• newer QT version does not play with the old windows image used in appveyor see also https://github.com/ilastik/ilastik/commit/1ee52d2400facc5a925edada3faddb52adee20af
• pytest doesn't like test files with same base names
• datasource factory for LazyflowSource now returns a CacheSource

[k-dominik]

where is the :partyparrot: when you need it?