Create cache merging tests
fix #1851 - merge popup information
Fixes #3000: Basic members caches missing on map

Fixes #3000 - a second, much simpler fix (my first attempt being pull
request #3055).

The problem was that caches loaded from the nearby list were being
initialized with zoom level (max zoom level + 1), or 19. As far as I can
tell there was no useful purpose for that, but it was preventing caches
loaded from the live map at any zoom level from ever over-writing the null
coordinates. So I made caches initialize with zoom level -1 instead.
Finetuning fixes for #1851 and #3000 (cache merging)