Define XDG_RUNTIME_DIR for Wayland socket and background Phoc in test

Disclaimer: First-time contributor here

Fixed a segfault in the tests related to XDG_RUNTIME_DIR not being set when Phoc is started. XDG_RUNTIME_DIR is required (See Running Weston) in order for the Wayland socket to be created.

Note: I backgrounded phoc in the test command so the CI environment didn't hang after phoc was initialized. Open to hearing any other suggestions that might not require backgrounding.

Edit: Looks like backgrounding may have just hidden the issue. Looking into this further...