[3.6] bpo-31027: Fix test_listcomps failure when run directly (GH-2939) (#2945)

Bug appears to be incomplete copy-paste-edit.
(cherry picked from commit ceb93f45)

Lib/test/test_listcomps.py

@@ -139,7 +139,7 @@ def test_main(verbose=None):
         import gc
         counts = [None] * 5
         for i in range(len(counts)):
-            support.run_doctest(test_genexps, verbose)
+            support.run_doctest(test_listcomps, verbose)
             gc.collect()
             counts[i] = sys.gettotalrefcount()
         print(counts)