Migrate Microsoft.Bot.Streaming.Tests project to xUnit

[denscollo]

**Note: This PR must be merged after #4590 **

Fixes #4106

Description

This PR migrates all Microsoft.Bot.Streaming.Tests tests from MSTest to xUnit.

Detailed Changes

• Removed MSTest packages from the .csproj.
• Replaced [TestMethod] with [Fact].
• Changed Asserts.
• Used constants instead of variables where applicable.
• Used var instead of the specific type.

Also, we did the following changes in:

• ConcurrentStreamTests:
  • Replaced CollectionAssert.AreEquivalent with Assert.Equal. This was possible because the arrays we compare are ordered.
• NamedPipeTransportTests:
  • Removed TestContext and replaced it with ITestOutputHelper.
  • Added a constructor to the test class and assigned the _output property.
  • Replaced TestContext.WriteLine with _output.WriteLine.

Testing

The following image shows the tests passing after the changes.