Update unit tests to Microsoft.CodeAnalysis.Testing

[sharwell]

I was looking at dotnet/roslyn-sdk#123 and considering what it might look like to switch from a source code distribution to a NuGet-based distribution. This is a proof of concept that preserves much of the original form used for testing StyleCop Analyzers, but separates the StyleCop Analyzers-specific items from the base testing support.

/cc @mavasani @DustinCampbell @jmarolf @JohanLarsson

[codecov]

Codecov Report

Merging #2751 into master will increase coverage by 0.01%.
The diff coverage is 99.56%.

@@            Coverage Diff             @@
##           master    #2751      +/-   ##
==========================================
+ Coverage   97.38%   97.39%   +0.01%     
==========================================
  Files         752      752              
  Lines       99590    97181    -2409     
  Branches     3267     3171      -96     
==========================================
- Hits        96982    94652    -2330     
+ Misses       1738     1680      -58     
+ Partials      870      849      -21

[JohanLarsson]

Why pass in explicit empty collection of diagnostic results when testing valid code?

await this.VerifyCSharpDiagnosticAsync(testCode, EmptyDiagnosticResults, CancellationToken.None).ConfigureAwait(false);

vs

await VerifyValidAsync(testCode).ConfigureAwait(false);

Also removing async:

VerifyValid(testCode);

[sharwell]

@vweijsters @pdelvo @JohanLarsson This is now ready for review. After getting a sense of the changes from looking at a few updated test classes, I recommend reviewing the more recent commits to understand the direction change. The merge commits are helpful as well, e.g. 7781c99 shows how the tests added in #2779 were updated to account for the new library.

[pdelvo]

I reviewed the main changes and took a look at some (not all) of the test classes. Those changes look good to me.