Add benchmarks mandelbrot and fannkuch (#12933)

Add benchmarks mandelbrot and fannkuch (#12933)

* Add benchmarks mandelbrot and fannkuch

Adding mandelbrot and fankuch-redux from Benchmarks Game to
complete the representation of benchmarks from this list
http://benchmarksgame.alioth.debian.org/u64q/csharp.html

* Update source acknolegments and edit return codes

* Add checksums to verify benchmark runs

This commit adds checksums, which are pre-computed for each input
case, to verify that the fannkuch and mandelbrot benchmarks
actualy compute the desired function

* Revert version bump

* Remove XunitPerfHarness from Main

* Remove an unnecessary line

* Add iterations to fannkuch to inscrease runtime