Add support for running/debugging NUnit tests

[JPAhnen]

For NUnit tests, Omnisharp-Roslyn returns a new feature name
"NUnitTestMethod". When a test is executed, the test framework name must
be set (xunit or nunit). The feature name for xUnit tests did not
change.

[dnfclas]

Hi @HexWrench, I'm your friendly neighborhood .NET Foundation Pull Request Bot (You can call me DNFBOT). Thanks for your contribution!

In order for us to evaluate and accept your PR, we ask that you sign a contribution license agreement. It's all electronic and will take just minutes. I promise there's no faxing. https://cla2.dotnetfoundation.org.

TTYL, DNFBOT;

[dnfclas]

@HexWrench, Thanks for signing the contribution license agreement so quickly! Actual humans will now validate the agreement and then evaluate the PR.

Thanks, DNFBOT;

[DustinCampbell]

This looks good, but I'll need to take a new build of OmniSharp with OmniSharp/omnisharp-roslyn#683 before I can merge this.

[JPAhnen]

As commented here, using this without the changes in Omnisharp-Roslyn won't break the xUnit functionality.

[DustinCampbell]

I understand, but I'll wait until I've got an OmniSharp server update before pulling this into our master branch here. Once it lights up properly, I'll push a new beta release.

[DustinCampbell]

Sorry it took so long to get an OmniSharp server update @HexWrench. Thanks very much for your contribution!