force test_merge_opt_runtime to run in FAST_COMPILE mode because inplace…

theano/tensor/tests/test_gc.py

@@ -73,7 +73,10 @@ def test_merge_opt_runtime():
         else:
             r = x
     t = time.time()
-    f = theano.function([x], r)
+    f = theano.function([x], r, mode='FAST_COMPILE')
+    # FAST_RUN does in-place optimizer which requires a lot of toposorting, which is actually
+    # pretty slow at the moment.  This test was designed to test MergeOptimizer... so I'm
+    # leaving toposort optimizations for a later date.
     dt = time.time() - t
 
     assert dt < 5.0 #it should never take longer than 5 seconds to compile this graph