Add an initial version of our testing suite

tests/README.md

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+# image2cpp test suite
+
+Test uses the real `index.html` file and scripts in a headless browser
+using `Playwright`. Output is compared against known valid (`golden`) files.
+This way we are testing the output of the actual tool, not a reimplementation.
+
+## Setup and running
+
+```
+python3 -m venv .venv
+source .venv/bin/activate
+pip install -r tests/requirements.txt
+playwright install chromium
+```
+
+To run:
+```
+python3 tests/test_conversions.py
+```
+
+Prints `ok`/`FAIL` per scenario and exits non-zero if anything doesn't
+match.
+
+## Updating golden files and test image
+
+To generate the test image again (only needed if we changed the image):
+```
+python3 tests/generate_test_image.py
+```
+
+After an intentional change to the conversion logic, regenerate the
+golden output and optionally review the diff before committing:
+
+```
+python3 tests/test_conversions.py --update
+git diff tests/golden
+```