tests: prioritised conf overrides and add invariant check

[miguelpires]

5e4242c changes our configuration files to use a numbered prefix. This stems from https://github.com/snapcore/snapd/pull/13967 which had a config override that was not being applied due to the file being sorted before the local.conf. This commit changes files created by the test harness to be formatted as 0X-.conf while test-specific overrides have higher numbers and are also named after the test (so we can find the culprit easily if an override is not removed). 054cfe3 adds an invariant check that fails if configuration overrides are left by tests

[miguelpires]

There's an issue in the invariant check but should be simple to fix. I'll do it next week and ping for reviews again

[miguelpires]

Put this on hold for now so I'll close it in the meantime. Thanks for the reviews