Add ocimage unittest

The mimeType tests fail, because of #4258.
I can't wrap my head around, what the issue is for `testToString()` and `testData()` …

I may also change the testgif, if I have time/am in a mood for it … The one I did hurts to look at :|

@DeepDiver1975 
@tanghus @icewind1991 @bartv2