Fix process reading code to try to avoid deadlocks

Author: andrewlock

tracer/test/Datadog.Trace.TestHelpers/ProcessHelper.cs

@@ -130,6 +130,8 @@ public virtual void Dispose(int waitForExitTimeout)
                     // Ignore exceptions when killing the process, as it may have already exited
                 }
             }
+
+            Process.Dispose();
         }
     }
 }