tests: set PIPEWIRE_RUNTIME_DIR and stop using XDG_RUNTIME_DIR
Using XDG_RUNTIME_DIR is problematic in test-reserve-device because GTestDBus also exports XDG_RUNTIME_DIR and then unsets it, which makes pipewire end up using $HOME for its socket. Using PIPEWIRE_RUNTIME_DIR is safer, as this variable is only used by pipewire.
Also use the build directory as a runtime dir (it always exists and nothing is going to interfere with it) and unset XDG_RUNTIME_DIR from the CI scripts, since nothing else is using it.
Closes #39 (closed)