Move cast from test class into the correct spot

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Avoid using SCIFIO in imagej-common's tests

Curtis says this is bad, bad, bad.

Actually, he said that the hierarchy is being restructured and that
imagej-common should be downstream of both SCIFIO and ImageJ-OPS, and it
all makes sense, but that is not half as funny a commit message.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>