BLD: test_perf.py gc disable can exhaust memory (where are the cycles?)

Author: y-p

vb_suite/test_perf.py

@@ -261,6 +261,7 @@ def profile_head_single(benchmark):
 
             results.append(d.get('timing',np.nan))
             gc.enable()
+            gc.collect()
 
     finally:
         gc.enable()