Turn ChunkExecutorBenchmark into a unit test

As some of the operations are ghastly expensive, let's enable them only
when the system property imagej.ops.expensive.tests is set to "enabled".

To see that this commit actually renamed the original file, inspect it
via with "git show -M40 --histogram".

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