[MXAPPS-581] Disable a long test in the SD nightly. (#12326)

* Disable a test that's taking longer than 10 minutes with the Python 2
  interpreter in the Straight Dope Nightly.

tests/nightly/straight_dope/test_notebooks_single_gpu.py

@@ -35,6 +35,7 @@
     'chapter02_supervised-learning/environment',
     'chapter03_deep-neural-networks/kaggle-gluon-kfold',
     'chapter04_convolutional-neural-networks/deep-cnns-alexnet',  # > 10 mins.
+    'chapter05_recurrent-neural-networks/rnns-gluon', # > 10 mins.
     'chapter06_optimization/gd-sgd-scratch',  # Overflow warning is intended.
     'chapter06_optimization/gd-sgd-gluon',  # Overflow warning is intended.
     'chapter07_distributed-learning/multiple-gpus-scratch',
@@ -176,9 +177,6 @@ def test_lstm_scratch(self):
     def test_gru_scratch(self):
         assert _test_notebook('chapter05_recurrent-neural-networks/gru-scratch')
 
-    def test_rnns_gluon(self):
-        assert _test_notebook('chapter05_recurrent-neural-networks/rnns-gluon')
-
     # Chapter 6
 
     def test_optimization_intro(self):