Maybe overriding `HOME` is not sufficient for faking a different home dir
to be picked up by `QStandardPaths`. So this change makes
`LIB_SYNCTHING_CONNECTOR_SYNCTHING_CONFIG_DIR` a hard override and uses it
in tests to fake a different path independently from the behavior of
`QStandardPaths`.
* Allow setting Syncthing path and port like it is possible with other
tests
* Test configuring currently running instance
* Avoid race condition; one must subscribe to `settingsChanged` before
applying changes