Add tests for dotnet/coreclrdotnet/corefx#20815 - new Math and MathF rounding modes...

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