fix: Ensure snap points always load

[DafyddLlyr]

Addresses https://trello.com/c/NZrvqg7d/2055-get-snap-points-on-map-to-always-load

Currently, points are automatically generated on the assumption that vector tiles will be loaded. On slower connections, this means that snap points either do not display or do not fully display. This can be mocked in the browser by throttling your connection speed.

This PR fixes this by waiting for the map loadend event before triggering the generation of the snap points. Originally I was listening for the vector source's tileloadend event but this fires for each tile load and then regenerates snap points which I imagine is both computationally expensive and could lead to inconsistent results (e.g. all snap points loading in one tile at a time).

Also - some basic OL tests! Couldn't quite get around a few event dispatch issues so I'm manually calling them currently. This would be great to try and remove - possibly something to tackle when/if we add Cypress to the stack.

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[DafyddLlyr]

Get snap points on map to always load

[jessicamcinchak]

This works great for me - snaps load a lot more consistently on a slow connection!

Thanks for tests too - there's some great foundational stuff there in addition to the snap-specific logic. A few minor comments but no blockers for me!