Karma is deprecated - change unit test library

[gavinr-maps]

Describe the problem

As of April 2023, Karma is deprecated, and we use Karma for our unit tests (see here / here / etc).

Describe the proposed solution

We should remove Karma and switch to a different unit testing library.

• Hopefully one that would require minimal changes to the actual unit test code.
• We probably want to follow what the Leaflet library does. They are still in progress of removing Karma too: Karma is deprecated Leaflet/Leaflet#8939 (as of 2025-01-14).
• After we choose something, we should also make similar changes in the other esri-leaflet-* repos like esri-leaflet-vector, esri-leaflet-geocoder, etc.

Additional Information

Related:

• Document how to write proper tests with Sinon+Chai+Karma for contributors #276

[gavinr-maps]

Discussed with @patrickarlt today - we would like to wait until the ArcGIS REST JS project decides on a way forward for this: Esri/arcgis-rest-js#1187 (comment)

... then we'll tackle it here.