Fixes #4088

Description

This PR migrates all Microsoft.Bot.Builder.AI.QnA.Tests project from MSTest to xUnit.

Specific Changes

• Replaced MSTest packages with xUnit in the .csproj.
• Replaced [TestMethod] with [Fact].
• Replaced [TestCategory] with [Trait].
• Changed Asserts
• QnAMakerRecognizerTests:
  • Created new class QnAMakerRecognizerFixture to implement IClassFixture for share code.
  • Replaced ClassInitialize with IClassFixture to share ResourceExplorer within tests.
  • Added testName argument to CreateFlow method to replace the TestContext.TestName MSTest feature.
• QnAMakerTests:
  • Added testName argument to CreateFlow method to replace the TestContext.TestName MSTest feature.
  • Removed unreachable code in QnaMaker_V2LegacyEndpoint_ConvertsToHaveIdPropertyInResult and renamed it as QnaMaker_V2LegacyEndpoint_Should_Throw
  • Removed unreachable code in QnaMaker_V3LegacyEndpoint_ConvertsToHaveIdPropertyInResult and renamed it as QnaMaker_V3LegacyEndpoint_Should_Throw
• Startup:
  • Replaced [AssemblyInitialize] with AssemblyFixture.
• Removed unused variables.
• Use constants instead of variables where applicable.
• Removed redundant parenthesis.

Testing

The following image shows the tests passing after the changes.