tests/bench: Add testcase for positional/kwargs to enumerate().

Inspired by discussion in #577. So, in this case of builtin function,
passing args by keyword has less than 1% overhead.

tests/bench/func_builtin-1-enum_pos.py

+import bench
+
+def test(num):
+    for i in iter(range(num//20)):
+        enumerate([1, 2], 1)
+
+bench.run(test)

tests/bench/func_builtin-2-enum_kw.py

+import bench
+
+def test(num):
+    for i in iter(range(num//20)):
+        enumerate(iterable=[1, 2], start=1)
+
+bench.run(test)