Tests for index request query parameters

[abarciauskas-bgse]

Add a tests for index parameters since its a bit of unclear how that is intended to work:

1. layer_dependency is a request parameter for tile endpoint - this is part of the titiler-stacapi-mspc factory.py
2. layer_dependency is of type AssetsBidxExprParams (which inherits both AssetsParams, BidxParams). As such, assets= or expression= is required.
3. When assets= is passed, those assets are used in tiling
4. If assets_indexes is passed, the value passed in the request args is then parsed to a dictionary via parse_asset_indexes
5. indexes could also be passed as part of the BidxParams class and this is used as a fall back

AssetsBidxExprParams is part of titiler core. So the functionality that should be tested here is just that when assets, asset_indexes and indexes are present in the request, they are passed as expected to the Reader.tile method in AssetsReader and not passed at all to the XarrayReader.tile method.