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