Factor out an abstract test base class

We actually need an OpService in pretty much all the tests. So let's have a base class.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Use AbstractOpTest as superclass in all of our tests

This simplifies the source code in src/test/ greatly.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Merge branch 'simplify-tests'

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