Disable unstable SELU test.
See https://github.com/keras-team/keras/issues/13856.

diff --git a/tests/keras/activations_test.py b/tests/keras/activations_test.py
index 4c0bb36..5808c0f 100644
--- a/tests/keras/activations_test.py
+++ b/tests/keras/activations_test.py
@@ -208,24 +208,6 @@ def test_elu():
     assert_allclose(result, true_result)
 
 
-def test_selu():
-    x = K.placeholder(ndim=2)
-    f = K.function([x], [activations.selu(x)])
-    alpha = 1.6732632423543772848170429916717
-    scale = 1.0507009873554804934193349852946
-
-    positive_values = get_standard_values()
-    result = f([positive_values])[0]
-    assert_allclose(result, positive_values * scale, rtol=1e-05)
-
-    negative_values = np.array([[-1, -2]], dtype=K.floatx())
-
-    result = f([negative_values])[0]
-    true_result = (np.exp(negative_values) - 1) * scale * alpha
-
-    assert_allclose(result, true_result)
-
-
 def test_tanh():
     test_values = get_standard_values()
 
-- 
2.25.1