Add tests for dotnet/coreclrdotnet/corefx#20815 - new Math and MathF rounding modes (dotnet/corefx#33297)
* add tests for new midpointrounding modes
new tests added for Math.cs double Round methods
* add tests for float, decimal
convert to theory
* Update tests
tests now reflect correct rounding behavior
* add tests for existing modes
* code review feedback
- refactor tests to use member data
- move MathF tests to MathF.netcoreapp.cs
- add tests for new modes with digits and special cases
* move math tests to mathtests.netcoreapp.cs
netcoreapp specific tests moved
Commit migrated from https://github.com/dotnet/corefx/commit/
cc42bb95b0877a653d4f556a0839d261ea3563b7