Verify doesn't allow diagnostics with the same id

[manfred-brands]

This is the same as #99 which was then solved by the writer to make all id's unique.

I ran into this again with another Analyzer.
Looking at the Roslyn analyzer they also use multiple DiagnosticsDescriptors with the same Id, just with different message/title.
See:

• IdentifiersShouldNotMatchKeywords

• DefineAccessorsForAttributeArguments

There are several others.

So I think the Unique test in RoslynAssert.cs VerifyAnalyzerSupportsDiagnostic should be removed.

Currently ExpectedDiagnostic only works on the RuleId, maybe it should work on DiagnosticDescriptor instead which then needs to be one listed in the SupportedDiagnostics.

[JohanLarsson]

This should also have been a separate pull request.

[manfred-brands]

I offered to split it off. Didn't expect it to be accepted straight away.

[JohanLarsson]

Ok, no worries.
Just wanted to put my foot down in case it was designed as hidden cargo in the PR :D