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/loop_count-3-while_up.py

@@ -0,0 +1,8 @@
+import bench
+
+def test(num):
+    i = 0
+    while i < num:
+        i += 1
+
+bench.run(test)