tests: functional: Don't try disable app after tests if app was not installed
Fixes an issue where app tests teardown fails if app installation fails or app is not available for distribution.
Tests:
- On Debian Bookworm, all miniflux functional tests are skipped and there are no errors.
- After running gitweb metadata functional test, the app is disabled.
Signed-off-by: Veiko Aasa veiko17@disroot.org