Implement IsSuppressed diagnostic validation via reflection.

src/Microsoft.CodeAnalysis.Testing/Microsoft.CodeAnalysis.Analyzer.Testing/AnalyzerTest`1.cs

@@ -15,6 +15,7 @@ using System.Threading.Tasks;
 using Microsoft.CodeAnalysis.Diagnostics;
 using Microsoft.CodeAnalysis.Host.Mef;
 using Microsoft.CodeAnalysis.Options;
+using Microsoft.CodeAnalysis.Testing.Extensions;
 using Microsoft.CodeAnalysis.Testing.Model;
 using Microsoft.CodeAnalysis.Text;
 using Microsoft.VisualStudio.Composition;
@@ -409,6 +410,19 @@ namespace Microsoft.CodeAnalysis.Testing
                         message);
                 }
 
+                if (expected.IsSuppressed.HasValue)
+                {
+                    if (actual.diagnostic.TryGetIsSuppressed(out var actualValue))
+                    {
+                        message = FormatVerifierMessage(analyzers, actual.diagnostic, expected, $"Expected diagnostic suppression state to match");
+                        verifier.Equal(expected.IsSuppressed.Value, actualValue, message);
+                    }
+                    else
+                    {
+                        throw new NotSupportedException("DiagnosticSuppressors are not supported on this platform.");
+                    }
+                }
+
                 DiagnosticVerifier?.Invoke(actual.diagnostic, expected, verifier);
             }
         }
@@ -803,6 +817,13 @@ namespace Microsoft.CodeAnalysis.Testing
                     builder.Append(")");
                 }
 
+                if (diagnostics[i].TryGetIsSuppressed(out var isSuppressed) && isSuppressed)
+                {
+                    builder.Append($".{nameof(DiagnosticResult.WithIsSuppressed)}(");
+                    builder.Append(isSuppressed);
+                    builder.Append(")");
+                }
+
                 builder.AppendLine(",");
             }
 
@@ -886,6 +907,13 @@ namespace Microsoft.CodeAnalysis.Testing
                     builder.Append(")");
                 }
 
+                if (diagnostics[i].IsSuppressed.HasValue)
+                {
+                    builder.Append($".{nameof(DiagnosticResult.WithIsSuppressed)}(");
+                    builder.Append(diagnostics[i].IsSuppressed.GetValueOrDefault());
+                    builder.Append(")");
+                }
+
                 builder.AppendLine(",");
             }
 

src/Microsoft.CodeAnalysis.Testing/Microsoft.CodeAnalysis.Analyzer.Testing/Extensions/DiagnosticExtensions.cs

@@ -0,0 +1,32 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+// See the LICENSE file in the project root for more information.
+
+using System;
+using System.Collections.Generic;
+using System.Reflection;
+using System.Text;
+
+namespace Microsoft.CodeAnalysis.Testing.Extensions
+{
+    internal static class DiagnosticExtensions
+    {
+        private static readonly MethodInfo? IsSuppressedGetMethod = typeof(Diagnostic).GetProperty("IsSuppressed")?.GetMethod;
+
+        public static bool TryGetIsSuppressed(this Diagnostic diagnostic, out bool value)
+        {
+            value = false;
+
+            var rawValue = IsSuppressedGetMethod?.Invoke(diagnostic, null);
+
+            if (rawValue?.GetType() != typeof(bool))
+            {
+                return false;
+            }
+
+            value = (bool)rawValue;
+
+            return true;
+        }
+    }
+}