Flaky test: Evaluator_Tests.VerifyDTDProcessingIsDisabled2

```
Microsoft.Build.UnitTests.Evaluation.Evaluator_Tests.VerifyDTDProcessingIsDisabled2
Shouldly.ShouldAssertException : _httpListenerThreadException\n should be null but was\nSystem.Net.HttpListenerException (0x80004005): The process cannot access the file because it is being used by another process\r\n at System.Net.HttpListener.AddAllPrefixes()\r\n at System.Net.HttpListener.Start()\r\n at Microsoft.Build.UnitTests.Evaluation.Evaluator_Tests.HttpServerThread() in C:\\MSBuild\\src\\Build.UnitTests\\Evaluation\\Evaluator_Tests.cs:line 4822
at Shouldly.ShouldBeNullExtensions.ShouldBeNull[T](T actual, Func`1 customMessage)
at Shouldly.ShouldBeNullExtensions.ShouldBeNull[T](T actual)
at Microsoft.Build.UnitTests.Evaluation.Evaluator_Tests.VerifyDTDProcessingIsDisabled2() in C:\MSBuild\src\Build.UnitTests\Evaluation\Evaluator_Tests.cs:line 4284
```