Go system tests: Close proxy and stub at the end of a test.

Fixes=#743

R=viettrungluu@chromium.org

Review URL: https://codereview.chromium.org/1861153002 .
diff --git a/mojo/go/tests/regression_test.go b/mojo/go/tests/regression_test.go
index 47c056c..e948f78 100644
--- a/mojo/go/tests/regression_test.go
+++ b/mojo/go/tests/regression_test.go
@@ -50,5 +50,7 @@
 			t.Fatal("proxy returned from the call before the stub sent the response")
 		}
 		v.mu.Unlock()
+		proxy.Close_Proxy()
+		stub.Close()
 	}
 }