Update tests using xunit to use nunit

[meaghanlewis]

Background:
We currently use both xUnit and NUnit test runners.

Xunit has certain problems such as silently crashing and leaves us uncertain about whether test actually executed or now.

Goal:
Change all the test the use xunit to use nunit instead.

Xunit test suites are:

• test/GitHub.InlineReviews.UnitTests
• test/UnitTests

[meaghanlewis]

PR for converting test/GitHub.InlineReivews.UnitTests: #1411

[meaghanlewis]

PR for converting test/UnitTest: #1418