tests: Format all Python code with black, except tests in basics subdir.

This adds the Python files in the tests/ directory to be formatted with
./tools/codeformat.py.  The basics/ subdirectory is excluded for now so we
aren't changing too much at once.

In a few places `# fmt: off`/`# fmt: on` was used where the code had
special formatting for readability or where the test was actually testing
the specific formatting.

tests/internal_bench/loop_count-4-while_down_gt.py

@@ -1,7 +1,9 @@
 import bench
 
+
 def test(num):
     while num > 0:
         num -= 1
 
+
 bench.run(test)