[Proposal] Analyzer test readability improvements

[Alex-Sob]

I would like to suggest a couple of changes that can improve readability of code analyzer tests:

• Use raw string literals for C# test source code in all unit tests for code analyzers.

• I would suggest to apply [StringSyntax("C#-test")] to all helper methods used to run analyzer tests, for example:

  public static class CSharpAnalyzerVerifier<TAnalyzer>
  {
    public static async Task VerifyAnalyzerAsync([StringSyntax("C#-test")] string source, params DiagnosticResult[] expected) { ... }
  }

  This should enable syntax highlighting for test C# source code in unit tests for analyzers. Currently the code in a test might look like this:

With raw string literals and StringSyntax attribute, this is what it will look like (I included such a test in #63641):

Also VS draws a nice vertical line for raw string literals, which also means that there's no extra whitespace before the code so that it does not have to start from the very left overflowing the test.