Split ImgTest into separate unit test classes

This test class previously resided in the imglib2-ij component for the
sole reason that it contained a unit test for the ImagePlusImg class.

This change splits apart the unit tests such that each Img subclass's
tests reside in their own unit testing class, as per normal unit testing
conventions. To share code, we migrate the common protected methods from
ImgTest into an ImgTestHelper class in imglib2 core. Then we split the
test method for each Img subclass into its own unit test class in the
appropriate component; i.e.: ArrayImg, CellImg and PlanarImg move to
imglib2 core, while ImagePlusImg remains in imglib2-ij.
Move non-imglib2-ij-specific manual test classes

These classes belong in imglib2-tests, not imglib2-ij.
That includes the ImgLib2 benchmark I wrote long ago, too.

Note that this change purges the mpicbg dependency fully from
imglib2-ij, freeing it from any shadow of the GPL, even in test scope!