Test data suit

It seems both Rust implementations of pmtiles got the spec wrong - based on my testing of pmtiles and pmtiles2 crates. I think the reason for that is that they both had to come up with their own test cases - thus obviously failing :)

In order to have the same behavior for all pmtiles implementations, we desperately need a standard set of test files that all implementations must pass (reminds me of the ACID browser tests.... memories...)

Anyway, could you standardize the tests by:

• consolidate all test data files in the https://github.com/protomaps/PMTiles/tree/main/js/test/data - or else it is unclear if they are the same or not? (I see some files in the /js dir)
• create a readme in that same dir that points to the specific test code in one of the implementations (no point of describing the tests in the markdown, as i am sure it will quickly out of sync with the actual unit tests)

The reason for all this is that Martin tile server has received a great bug report https://github.com/maplibre/martin/issues/675 (thx @tobinbradley !) that highlights a bug, but I have no good way of converting that 200MB file into a useful pmtiles test file - so was hoping to find a test fixture here :)

Once again, thanks @bdon !

Start by inspecting js/test/data and the other test-data files under /js, then compare them with the test code in the referenced pmtiles implementations and the Martin bug report. Consolidate the fixtures in js/test/data and add a README there linking to the specific implementation test code; done means the shared files and their authoritative test reference are clear.