Improve support for DiagnosticSuppressor

[Flash0ver]

I just tried writing unit tests for a DiagnosticSuppressor via Microsoft.CodeAnalysis.Testing, but I have not succeeded.
Although DiagnosticSuppressor does derive from DiagnosticAnalyzer, and with that already inheriting support in the available infrastructure of Microsoft.CodeAnalysis.Testing, I was still unable to get it working.

My approach was to always add a custom DiagnosticAnalyzer which reports all diagnostics defined in the Assert-Phase of the test through a custom collection of DiagnosticResults.
After this Testing-Internal DiagnosticAnalyzer has reported all arranged diagnostics, the DiagnosticSuppressor under test could then suppress these, so that there are no ExpectedDiagnostics left.
But I couldn't really get the Markup-Syntax working, nor manually setting the Location/Span of the arranged diagnostic(s) to be suppressed during the Act-Phase, so that the Assert-Phase can expect zero ExpectedDiagnostics, or at least a set of ExpectedDiagnostics without the previously reported and suppressed DiagnosticResults. Maybe I got something wrong with the Location/TextSpan/Path: the DiagnosticSuppressor did receive the ReportedDiagnostics, but even after invoking ReportSuppression the test still failed with the message "Mismatch between number of diagnostics returned, expected "0" actual "1"".

Perhaps adding such a DiagnosticResult-Collection to the AnalyzerTest`1 or ProjectState (or another suitable type), which is iterated and diagnostics produced from, which are then subject to be passed to the DiagnosticSuppressor under test if supported, would be of great benefit for all authors of analyzers utilizing the might of Microsoft.CodeAnalysis.Testing.
I guess there is also a different perhaps more intuitive approach to be discussed.

[sharwell]

I was expecting this to be fixed by #917. Have you had a chance to try this functionality out?

[Flash0ver]

Thank you @sharwell for the hint. I tried it out and it worked perfectly.
Thank you @tom-englert for the implementation and the great example!