Dynamically build test fixtures

@maxrjones pointed out [here](https://github.com/developmentseed/titiler-multidim/pull/101#discussion_r2453046106) that it would be better to dynamically generate test fixtures instead of committing them to the repo.

This would require some more work, since the expected responses would also have to be generated based on the new data, or we would need to switch from random data to something deterministic.