TST: disable gc in refcount test (numpy#11158)

The vectorize version of this test was failing consistently on several of the Appveyor builds, ever since a recent pytest upgrade.

Our theory is that by random chance, things changed so that during the call to vectorize(op).__call__, python started running a garbage collection, which perturbed the refcounts that this test is checking. (Specifically this test is doing a weird thing and checking that the refcount of the object 1 doesn't decrease, and it's very plausible that some random bit of garbage was holding a reference to this object.)

Disabling the gc during the test makes this kind of refcount assertion more reliable, and seems to have fixed the appveyor builds, so I guess it's good.

numpy/testing/_private/utils.py

@@ -1373,16 +1373,20 @@ def _assert_valid_refcount(op):
     """
     if not HAS_REFCOUNT:
         return True
-    import numpy as np
+    import numpy as np, gc
 
     b = np.arange(100*100).reshape(100, 100)
     c = b
     i = 1
 
-    rc = sys.getrefcount(i)
-    for j in range(15):
-        d = op(b, c)
-    assert_(sys.getrefcount(i) >= rc)
+    gc.disable()
+    try:
+        rc = sys.getrefcount(i)
+        for j in range(15):
+            d = op(b, c)
+        assert_(sys.getrefcount(i) >= rc)
+    finally:
+        gc.enable()
     del d  # for pyflakes