Nunit: Culture differences on .NET Core on non-Windows causes test failures

(@nunit/framework-team We'll need to take care of this before 3.10 or else pull the .NET Standard 2.0 build.)

These tests are Windows-specific; .NET Core on other operating systems causes them to fail. Mono, however, passes the tests so it seems to produces the Windows behavior with our .NET Framework DLLs on other OSs. Our .NET Standard DLLs on .NET Core are inconsistent with our .NET Framework DLLs on Mono.

https://github.com/dotnet/corefx/issues/11221 says the date format should be up to the OS by design.
https://github.com/dotnet/corefx/issues/17052 looks like it'll have the same conclusion for the string comparisons.

• Should we take the same stance as .NET Core and expect different results on Windows and Ubuntu?

• Should we take the same stance as (I think) Mono does and try to polyfill the WIndows behavior onto other OSs somehow (though this seems like low ROI)?

• We haven't yet done the work to enable PlatformAttribute on .NET Standard. Should we take the easy out for 3.10 and remove NUnit's culture APIs, or look into adding both platform-detection and culture APIs?

Failed : NUnit.Framework.Attributes.SetCultureAttributeTests.UseWithParameterizedTest()
  Expected string length 19 but was 17. Strings differ at index 6.
  Expected: "01.06.2010 00:00:00"
  But was:  "01.06.10 00:00:00"
  -----------------^

Failed : NUnit.Framework.Constraints.SubstringConstraintTests.SpecifyComparisonType(" ss ","ß",CurrentCulture,True)
  Expected: String containing "ß"
  But was:  " ss "

Failed : NUnit.Framework.Constraints.SubstringConstraintTests.SpecifyComparisonType(" ss ","ß",CurrentCultureIgnoreCase,True)
  Expected: String containing "ß"
  But was:  " ss "

Failed : NUnit.Framework.Constraints.SubstringConstraintTests.SpecifyComparisonType(" SS ","ß",CurrentCultureIgnoreCase,True)
  Expected: String containing "ß"
  But was:  " SS "

Failed : NUnit.Framework.Constraints.SubstringConstraintTests.SpecifyComparisonType(" ss ","ß",InvariantCulture,True)
  Expected: String containing "ß"
  But was:  " ss "

Failed : NUnit.Framework.Constraints.SubstringConstraintTests.SpecifyComparisonType(" ss ","ß",InvariantCultureIgnoreCase,True)
  Expected: String containing "ß"
  But was:  " ss "

Failed : NUnit.Framework.Constraints.SubstringConstraintTests.SpecifyComparisonType(" SS ","ß",InvariantCultureIgnoreCase,True)
  Expected: String containing "ß"
  But was:  " SS "