Add pause between computing lastModified date and executing task (#27073)

* Use FileSystemWatcher to check if file was modified

* Update ConcatenateFilesTest.cs

* Update ConcatenateFilesTest.cs

* Update ConcatenateFilesTest.cs

* Update ConcatenateFilesTest.cs

* Update ConcatenateFilesTest.cs

* Update ConcatenateFilesTest.cs

* Use Task.Delay

Author: captainsafia

src/Razor/Microsoft.NET.Sdk.Razor/test/ConcatenateFilesTest.cs

@@ -325,7 +325,7 @@ public void BundlesScopedCssFiles_DoesNotOverrideBundleForSameContents()
 
         [Fact]
         [QuarantinedTest("https://github.com/dotnet/aspnetcore/issues/25623")]
-        public void BundlesScopedCssFiles_UpdatesBundleWhenContentsChange()
+        public async System.Threading.Tasks.Task BundlesScopedCssFiles_UpdatesBundleWhenContentsChange()
         {
             // Arrange
             var expectedFile = Path.Combine(Directory.GetCurrentDirectory(), $"{Guid.NewGuid():N}.css");
@@ -382,14 +382,14 @@ public void BundlesScopedCssFiles_UpdatesBundleWhenContentsChange()
                     }),
             };
 
+            await System.Threading.Tasks.Task.Delay(1000);
             taskInstance.Execute();
 
             // Assert
             Assert.True(result);
             Assert.True(File.Exists(expectedFile));
             var actualContents = File.ReadAllText(expectedFile);
             Assert.Equal(UpdatedBundleContent, actualContents, ignoreLineEndingDifferences: true);
-
             Assert.NotEqual(lastModified, File.GetLastWriteTimeUtc(expectedFile));
         }
     }