tests/bench: Add testcase to compare bytes(N) vs b"\0" * N.

Based on python-dev discussion regarding PEP467.

tests/bench/bytealloc-1-bytes_n.py

+import bench
+
+def test(num):
+    for i in iter(range(num // 1000)):
+        bytes(10000)
+
+bench.run(test)

tests/bench/bytealloc-2-repeat.py

+import bench
+
+def test(num):
+    for i in iter(range(num // 1000)):
+        b"\0" * 10000
+
+bench.run(test)