ignore perf tests that slow down C# coverage on windows

src/csharp/Grpc.Core.Tests/PInvokeTest.cs

@@ -74,6 +74,8 @@ namespace Grpc.Core.Tests
         /// (~110ns .NET Windows)
         /// </summary>
         [Test]
+        [Category("Performance")]
+        [Ignore("Prevent running on Jenkins")]
         public void NativeCallbackBenchmark()
         {
             OpCompletionDelegate handler = Handler;
@@ -95,6 +97,8 @@ namespace Grpc.Core.Tests
         /// (~1.1us on .NET Windows)
         /// </summary>
         [Test]
+        [Category("Performance")]
+        [Ignore("Prevent running on Jenkins")]
         public void NewNativeCallbackBenchmark()
         {
             counter = 0;
@@ -112,6 +116,8 @@ namespace Grpc.Core.Tests
         /// (~46ns .NET Windows)
         /// </summary>
         [Test]
+        [Category("Performance")]
+        [Ignore("Prevent running on Jenkins")]
         public void NopPInvokeBenchmark()
         {
             BenchmarkUtil.RunBenchmark(