Allow flaky attribute on class and assembly (dotnet/extensions#1742)

* Allow flaky attribute on class and assembly\n\nCommit migrated from 23ba460e27

src/ObjectPool/test/DefaultObjectPoolTest.cs

@@ -45,7 +45,7 @@ namespace Microsoft.Extensions.ObjectPool
         {
             // Arrange
             var pool = new DefaultObjectPool<List<int>>(new ListPolicy());
-            
+
             // Act
             var list = pool.Get();
 

src/Testing/src/xunit/FlakyAttribute.cs

@@ -50,7 +50,7 @@ namespace Microsoft.AspNetCore.Testing.xunit
     /// </para>
     /// </example>
     [TraitDiscoverer("Microsoft.AspNetCore.Testing.xunit.FlakyTestDiscoverer", "Microsoft.AspNetCore.Testing")]
-    [AttributeUsage(AttributeTargets.Method)]
+    [AttributeUsage(AttributeTargets.Method | AttributeTargets.Class | AttributeTargets.Assembly)]
     public sealed class FlakyAttribute : Attribute, ITraitAttribute
     {
         /// <summary>