Microsoft.ML.InternalCodeAnalyzer should use Microsoft.CodeAnalysis.Testing for harness setup

[mareklinka]

As discussed in in this PR, the tests for the internal code analyzer should make use of the facilities provided in Microsoft.CodeAnalysis.Testing.

That would make the test project more best-practice compliant and easier to maintain.

[sharwell]

@shauheen This says assigned but I'm not sure who it's assigned to. I'm happy to work with the assignee to set up the tests and understand why things fail when they do. Some of the failure messages that appear are not at all helpful, so it's better to ask early when something fails. I'm working to update the messages for these situations but we have a long way to go.

[shauheen]

@sharwell , I had assumed that @mareklinka was interested in working on this, but as I don't see a PR. Please feel free to take it away. Please let me know so that I put assigned back.

[mareklinka]

@shauheen yes, sorry, I don't think I'll have the time to work on this in the near future.

[shauheen]

Thanks @mareklinka , no worries.