Add missing test data README to release assets and inventory all test files

[LOCEANlloydizard]

Following @leewujung’s suggestion in PR #1562:

1. Include the echopype/test_data/README.md file in the release assets for reference.
2. Perform an inventory of all test data files used across the repo and releases, since the previous README was incomplete.

Take the opportunity to address #716, #760, #648, #666