Update xUnit

[MaxAtoms]

In xUnit the class AssertActualExpectedException does not seem to exist anymore.
We therefore get a build error in Step.cs with XUnit Version 2.6.6.

Todo: Find out how we can obtain the necessary information so that we can guide the user like before.

[Sricharan-Anugandula]

Hi,
For this when we upgrade to latest version of Xunit ,AssertActualExpectedException does not exist anymore . For this problem we can use XunitException instead of AssertActualExpectedException .

New XunitException does not contain actual and expected properties like before so we can directly use Message property of
XunitException which gives almost same functionality as before.

For this I am attaching my PR
https://github.com/DotNetKoans/DotNetKoans/pull/200/commits