Improve Stopwatch DebuggerDisplayAttribute test (#70394)

src/libraries/System.Runtime.Extensions/tests/System/Diagnostics/Stopwatch.cs

@@ -99,22 +99,15 @@ public static void StartNewAndRestart()
         [Fact]
         public static void DebuggerAttributesValid()
         {
-            DebuggerAttributes.ValidateDebuggerDisplayReferences(new Stopwatch());
-
             Stopwatch watch = new Stopwatch();
-            Assert.Equal("00:00:00 (IsRunning = False)", GetDebuggerDisplayProperty(watch));
+            Assert.Equal("00:00:00 (IsRunning = False)", DebuggerAttributes.ValidateDebuggerDisplayReferences(watch));
             watch.Start();
             Thread.Sleep(10);
-            Assert.Contains("(IsRunning = True)", GetDebuggerDisplayProperty(watch));
-            Assert.DoesNotContain("00:00:00 ", GetDebuggerDisplayProperty(watch));
+            Assert.Contains("(IsRunning = True)", DebuggerAttributes.ValidateDebuggerDisplayReferences(watch));
+            Assert.DoesNotContain("00:00:00 ", DebuggerAttributes.ValidateDebuggerDisplayReferences(watch));
             watch.Stop();
-            Assert.Contains("(IsRunning = False)", GetDebuggerDisplayProperty(watch));
-            Assert.DoesNotContain("00:00:00 ", GetDebuggerDisplayProperty(watch));
-
-            static string GetDebuggerDisplayProperty(Stopwatch value)
-            {
-                return (string)typeof(Stopwatch).GetProperty("DebuggerDisplay", BindingFlags.Instance | BindingFlags.NonPublic).GetValue(value);
-            }
+            Assert.Contains("(IsRunning = False)", DebuggerAttributes.ValidateDebuggerDisplayReferences(watch));
+            Assert.DoesNotContain("00:00:00 ", DebuggerAttributes.ValidateDebuggerDisplayReferences(watch));
         }
 
         [OuterLoop("Sleeps for relatively long periods of time")]