Ignore suppressed diagnostics in code fix verifier (#88492)

The failures started with
299a8c9,
which pulled in changes to the test SDK such that suppressed warnings
are now included in the set of warnings checked by the code fix
verifier.

src/tools/illink/test/ILLink.RoslynAnalyzer.Tests/Verifiers/CSharpCodeFixVerifier`2.cs

@@ -2,6 +2,8 @@
 // Licensed under the MIT license. See LICENSE file in the project root for full license information.
 
 using System.Collections.Generic;
+using System.Collections.Immutable;
+using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 using ILLink.Shared;
@@ -36,6 +38,15 @@ public Test ()
 					return solution;
 				});
 			}
+
+			protected override ImmutableArray<(Project project, Diagnostic diagnostic)> SortDistinctDiagnostics (IEnumerable<(Project project, Diagnostic diagnostic)> diagnostics)
+			{
+				// Only include non-suppressed diagnostics in the result. Our tests suppress diagnostics
+				// with 'UnconditionalSuppressMessageAttribute', and expect them not to be reported.
+				return base.SortDistinctDiagnostics (diagnostics)
+					.Where (diagnostic => !diagnostic.diagnostic.IsSuppressed)
+					.ToImmutableArray ();
+			}
 		}
 
 		/// <inheritdoc cref="AnalyzerVerifier{TAnalyzer}.Diagnostic()"/>