[#4103] Migrate Microsoft.Bot.Configuration.Tests to xUnit

[denscollo]

Fixes #4103

Description

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

Specific Changes

• Replaced MSTest packages with xUnit in the .csproj.
• Changed [TestMethod] to [Fact].
• Changed Assert.AreEqual to Assert.Equal.
• Changed Assert.IsNotNull to Assert.NotNull.
• Changed Assert.IsNull to Assert.Null.
• Changed Assert.Fail to throw new XunitException.
• Changed [ExpectedException] to Assert.ThrowsAsync.
• Improved tests Assertions.

Testing

In the following image there are the passing tests.