Issue #1212 - use mono with dotnet-xunit if not on windows

[TheAngryByrd]

This closes #1212 .

This is just an initial stab at the solution. Feedback is welcome.

On OSX:

dotnet xunit -internaldiagnostics

Running desktop CLR tests for framework net452...
EXEC: "mono64" "/Users/jimmy/.nuget/packages/dotnet-xunit/2.3.0-jimmy001/tools/net452/xunit.console.exe" "MyCoolNewLib.Tests.dll" -internaldiagnostics
xUnit.net Console Runner (64-bit Desktop .NET 4.0.30319.42000)
  Discovering: MyCoolNewLib.Tests
  Discovered:  MyCoolNewLib.Tests
  Starting:    MyCoolNewLib.Tests
  Finished:    MyCoolNewLib.Tests
=== TEST EXECUTION SUMMARY ===
   MyCoolNewLib.Tests  Total: 1, Errors: 0, Failed: 0, Skipped: 0, Time: 0.109s

dotnet xunit -internaldiagnostics -x86

Running desktop CLR tests for framework net452...
EXEC: "mono" "/Users/jimmy/.nuget/packages/dotnet-xunit/2.3.0-jimmy001/tools/net452/xunit.console.x86.exe" "MyCoolNewLib.Tests.dll" -internaldiagnostics
xUnit.net Console Runner (32-bit Desktop .NET 4.0.30319.42000)
  Discovering: MyCoolNewLib.Tests
  Discovered:  MyCoolNewLib.Tests
  Starting:    MyCoolNewLib.Tests
  Finished:    MyCoolNewLib.Tests
=== TEST EXECUTION SUMMARY ===
   MyCoolNewLib.Tests  Total: 1, Errors: 0, Failed: 0, Skipped: 0, Time: 0.100s

[dnfclas]

This seems like a small (but important) contribution, so no Contribution License Agreement is required at this point. We will now review your pull request.
Thanks,
.NET Foundation Pull Request Bot

[bradwilson]

Thanks!