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.

tools/codeformat.py

@@ -50,6 +50,7 @@ PATHS = [
     "ports/**/*.py",
     "py/**/*.py",
     "tools/**/*.py",
+    "tests/**/*.py",
 ]
 
 EXCLUSIONS = [
@@ -57,6 +58,10 @@ EXCLUSIONS = [
     "ports/*/build*",
     # gitignore in ports/unix ignores *.py, so also do it here.
     "ports/unix/*.py",
+    # not real python files
+    "tests/**/repl_*.py",
+    # needs careful attention before applying automatic formatting
+    "tests/basics/*.py",
 ]
 
 # Path to repo top-level dir.