TST: test openly accessible tiles and mark broken with status="broken" flag

[martinfleis]

Splitting test suite into 2 files and adding tests that check all providers that do not require any API key.

I will also add a machinery marking broken sources coming from leaflet-providers and removing them from the compressed json. Just want to run CI before that to show how it behaves with broken tiles.

Some (Stadia) do not require API key according to our metadata but the domain needs to be whitelisted to make them work. Those give 403 error and are xfailed.

[martinfleis]

As shown in CI (https://github.com/geopandas/xyzservices/pull/78/checks?check_run_id=3274570828), we have a few broken sources:

• OpenPtMap OpenPtMap no longer exists leaflet-extras/leaflet-providers#388
• Hydda maps OpenStreetMap Sweden is down leaflet-extras/leaflet-providers#351
• nlmaps.luchtfoto nlmaps.luchtfoto phased out  leaflet-extras/leaflet-providers#387
• NASAGIBS.ModisTerraSnowCover NASAGIBS.ModisTerraSnowCover probably broken leaflet-extras/leaflet-providers#389

I will not remove them but add status="broken" attribute to them. That way we do not deviate from the original leaflet-providers set but clearly mark that they are broken. Since all are now reported it is likely that it will be resolved in the upstream repo.