Need new Mapbox Tilejson Unit tests

**Is your feature request related to a problem? Please describe.**
PR #449 had to remove a stale URL that was a public "TileJSON" endpoint for MapBox Vector Tiles in the Dutch SDI PDOK:
https://geodata.nationaalgeoregister.nl/beta/topotiles-viewer/styles/tilejson.json
There is no direct replacement URL as PDOK has changed to OGC API Vector Tiles APIs.

**Describe the solution you'd like**
Somehow we need an endpoint to test the GHC Probe `GeoHealthCheck.plugins.probe.mapbox.TileJSON`.

**Describe alternatives you've considered**
MapTiler has some public endpoints but these require a key. Maybe we can still use.

**Additional context**
The unit tests were removed by removing these fixtures from `tests/data/fixtures.json`:

```
In fixtures.json

Resource
"PDOK MAPBOX TILEJSON": {
"owner": "admin",
"resource_type": "Mapbox:TileJSON",
"active": true,
"title": "PDOK BRT+BGT Mapbox TileJSON",
"url": "https://geodata.nationaalgeoregister.nl/beta/topotiles-viewer/styles/tilejson.json",
"tags": [
"pdok",
"tiling"
]
}
Probe vars
"PDOK - MAPBOX TILEJSON": {
"resource": "PDOK MAPBOX TILEJSON",
"probe_class": "GeoHealthCheck.plugins.probe.mapbox.TileJSON",
"parameters": {
"lat_4326": "52",
"lon_4326": "5"
}
}
Check vars

"PDOK MAPBOX TILEJSON - HTTP - NoError": {
"probe_vars": "PDOK - MAPBOX TILEJSON",
"check_class": "GeoHealthCheck.plugins.check.checks.HttpStatusNoError",
"parameters": {}
},

```