Add validation step to integration tests Github Action

[robbibt]

This PR adds a new validation test step to our integration tests so that each time the integration test is run, the Coastlines data produced is also used to run a small-scale validation of the results by comparing them to validation data from the Narrabeen-Collaroy Beach Survey Program. The ensures that the code both works, and generates sensible results.

The Github action does the following:

1. Runs integation test that generates Coastlines data for Narrabeen Beach
2. Validates the Coastlines data against validation data
3. Exports a PNG image and a markdown file containings stats on the latest run
4. Commits those files back into the repo so they're visible in the tests/README.md file

Some other small improvements include:

• Moving all docker-compose test actions and file copying to a single shell script (docker/coastline/run_tests) for easier maintenance
• Move some docker-compose files for updating the test database into the docker subfolder
• Allow the buffer on the edge of scenes to be customised, so that the integration tests don't need to load more data than they need