Change default severity

DiagnosticVerifier.Helper.cs

var compilation = project.GetCompilationAsync().Result;
var specificDiagnosticOptions = new[] { new KeyValuePair<string, ReportDiagnostic>(NotReportedByDefaultAnalyzer.DiagnosticId, ReportDiagnostic.Warn) };
var options = new CSharpCompilationOptions(outputKind: OutputKind.DynamicallyLinkedLibrary, specificDiagnosticOptions: specificDiagnosticOptions);
var compilationWithOptions = compilation.WithOptions(options);
var compilationWithAnalyzers = compilationWithOptions.WithAnalyzers(ImmutableArray.Create(analyzer));
var diags = compilationWithAnalyzers.GetAnalyzerDiagnosticsAsync().Result;