fix: small fixup to new /ip test case (#214)

Realized that the test added in #213 had a couple of small-ish issues:
- Missing `t.Parallel()` call, unnecessarily slowing down overall test
  suite
- No need to spin up new ephemeral test server, prefer the existing
  shared one

Author: mccutchen

httpbin/handlers_test.go

@@ -412,8 +412,7 @@ func TestIP(t *testing.T) {
 		// connections but not for direct ServeHTTP calls as the used in the
 		// httptest.NewRecorder tests above, so we need to use a real server
 		// to verify handling of both cases.
-		srv := httptest.NewServer(app)
-		defer srv.Close()
+		t.Parallel()
 
 		resp, err := client.Get(srv.URL + "/ip")
 		assert.NilError(t, err)