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/feature_check/reverse_ops.py

@@ -1,8 +1,8 @@
 class Foo:
-
     def __radd__(self, other):
         pass
 
+
 try:
     5 + Foo()
 except TypeError: