Add a test-scope dependency on junit-benchmarks

This framework taps into JUnit and allows to make beautiful graphs and
performance regression tests based on records of previous runs.

To accomodate junit-benchmarks, we have to upgrade the junit dependency
to version 4.11.

Since upstream junit-benchmarks has a bug where it includes all @Before
and @After methods in the timing, we use our own forked version, as per
pom-scijava.

For a junit-benchmarks tutorial, see:

	http://labs.carrotsearch.com/junit-benchmarks-tutorial.html

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Turn AddOpBenchmark into a JUnit Benchmark

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Turn the MapperBenchmark into a JUnit Benchmark

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