tests: the coverage tests are not guaranteed to check firmware loading from custom path

In the integration test which run the binary directly (in daemon/tests), the setup copies the `.bit.bin` to `/lib/firmware` which means that when running a test to load the overlay, there is no guarantee that the firmware subsystem is looking in our custom firmware lookup path. We should only copy to `/lib/firmware` before the relevant tests, and we should delete after.

Also to check: that the firmware path is cleared between tests, otherwise we could be loading, e.g., "k26-starter-kits.bit.bin" from anywhere.