Tests fail due to reliance of system installed gschema file

The test suite fails if the system doesn't already have gnome-weather-3.31 or newer installed, because then it doesn't find the org.gnome.Weather schema (either because it's not installed yet, or because older versions had a org.gnome.Weather.Application schema instead).

For the purpose of tests, there should be a gschema compiled for tests-only use and pointed at it, to be independent of what happens or doesn't happen to be installed on the system. Various other projects manage it - gnome-weather should too.

Additionally I'm concerned about changes like this org.gnome.Weather.Application to org.gnome.Weather to the schema name, as that lost users previous settings, but that ship has sailed.