tests: Rename "bench" tests to "internal_bench" and run-internalbench.py

To emphasise these benchmark tests compare the internal performance of
features amongst themselves, rather than absolute performance testing.

tests/internal_bench/bytealloc-2-repeat.py

@@ -0,0 +1,7 @@
+import bench
+
+def test(num):
+    for i in iter(range(num // 1000)):
+        b"\0" * 10000
+
+bench.run(test)