tests: pin how the dev tools take their arguments

Both tools above used to crash, or write into a flag-named path, instead of
asking for what they need. These tests run them in an empty folder and check
that they exit non-zero with a usage line and write nothing.

🤖 Generated with Codebuff
Co-Authored-By: Codebuff <[email protected]>

tests/test_dev_tool_usage.py

@@ -0,0 +1,48 @@
+"""Dev tools that take paths ask for them, instead of crashing or writing a flag as a directory."""
+
+import subprocess
+import sys
+import tempfile
+import unittest
+from pathlib import Path
+
+TOOLS = Path(__file__).resolve().parents[1] / "tools"
+
+
+def run(name: str, *arguments: str) -> tuple[subprocess.CompletedProcess, list[str]]:
+    """Runs one tool in an empty folder; returns its result and whatever it left there."""
+    with tempfile.TemporaryDirectory() as folder:
+        result = subprocess.run([sys.executable, str(TOOLS / name), *arguments],
+                                cwd=folder, text=True, capture_output=True)
+        left = sorted(path.name for path in Path(folder).iterdir())
+    return result, left
+
+
+class MissingArgumentTests(unittest.TestCase):
+    def assert_asked_for_arguments(self, name: str, *arguments: str) -> None:
+        result, left = run(name, *arguments)
+        self.assertNotIn("Traceback", result.stderr, f"{name} crashed instead of printing usage")
+        self.assertNotEqual(result.returncode, 0, f"{name} reported success without its arguments")
+        self.assertIn("Usage", result.stderr + result.stdout, f"{name} printed no usage")
+        self.assertEqual(left, [], f"{name} wrote files without being told where")
+
+
+class JsonLoadFixturesTests(MissingArgumentTests):
+    def test_no_directory_asks_for_one(self):
+        self.assert_asked_for_arguments("json_load_fixtures.py")
+
+    def test_a_flag_is_not_an_output_directory(self):
+        # It used to create ./--help and fill it with 66 MB of fixtures.
+        self.assert_asked_for_arguments("json_load_fixtures.py", "--help")
+
+
+class BakeBricolageTests(MissingArgumentTests):
+    def test_no_fonts_asks_for_them(self):
+        self.assert_asked_for_arguments("bake_bricolage.py")
+
+    def test_one_font_is_not_enough(self):
+        self.assert_asked_for_arguments("bake_bricolage.py", "source.ttf")
+
+
+if __name__ == "__main__":
+    unittest.main()