Add tests for snapshots with different extensions

[mcol]

This adds two tests that pass on current main to help having a baseline for #2029. Following the discussion I've opened in #2106, we may add another test too (plus any other that may be suggested).

[hadley]

Thanks for getting started on this — as it looked the tests I remembered that a lot of these differences are due to the slightly different paths that expect_snapshot() and expect_snapshot_file() generate so I've tweaked the tests to hopefully make that more clear.