Fix running and debugging NUnit tests

[dgileadi]

I found that when I click the "run test" item on my NUnit test class, it wouldn't do anything. Turning up the log level showed me the call to the server:

[dbug]: OmniSharp.Middleware.LoggingMiddleware
        ************ Request ************
POST - /v2/runtest
************ Headers ************
Content-Type - application/json
************  Body ************
{
  "FileName": "/Users/gileadis/Projects/Armor of God/Unity/Armor of God/Assets/Scripts/Editor/TypewriterTextTests.cs",
  "MethodName": "TypewriterTextTests.TestComplete",
  "TestFrameworkName": "xunit"
}

Notice the TestFrameworkName of xunit, even though I'm running an NUnit test. Digging through the code pointed me to this typo. I'm not certain it's the culprit, but it seems worth fixing in any case.

[DustinCampbell]

Very nice catch!

[DustinCampbell]

This likely fixes #1635.