Move S.T.M. Span<char> tests to StringTests.cs(part 2) (dotnet/corefx…

…#29935)

* move s.t.m. tests

* fix netfx incompatibility

* fix linux EndsWithNoMatch_Char test

* rename to emptySlice

* use ReadOnlySpan<char> instead of var

* nit extra line

* add asserts for Contains

* add comment for difference between Windows and Unix

* address PR feedback

* add netfx to Configurations.props

* move netfx failing tests to String.netcoreapp.cs

* cleanup project files

* cleanup

* cleanup

* address PR feedback

* revert Contains tests

* merge with master

* fix project refs


Commit migrated from dotnet/corefx@3755c16