tests/basics/set_pop: Improve coverage of set functions.

tests/basics/set_pop.py

@@ -7,3 +7,12 @@ except KeyError:
 else:
     print("Failed to raise KeyError")
 
+# this tests an optimisation in mp_set_remove_first
+# N must not be equal to one of the values in hash_allocation_sizes
+N = 11
+s = set(range(N))
+while s:
+    print(s.pop()) # last pop() should trigger the optimisation
+for i in range(N):
+    s.add(i) # check that we can add the numbers back to the set
+print(list(s))