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

.gitlab-ci.yml

@@ -116,7 +116,6 @@ include:
     - ninja -C "$PW_BUILD_DIR" install
     # misc environment only for wireplumber
     - export WP_BUILD_DIR="$PWD/build-wireplumber-$BUILD_ID"
-    - export XDG_RUNTIME_DIR="$(mktemp -p $PWD -d xdg-runtime-XXXXXX)"
     - export PKG_CONFIG_PATH="$(dirname $(find "$PREFIX" -name 'libpipewire-*.pc')):$PKG_CONFIG_PATH"
   script:
     # Build wireplumber

tests/meson.build

@@ -30,6 +30,9 @@ endif
 
 # The common test environment
 common_test_env = environment({
+  'HOME': '/invalid',
+  'XDG_RUNTIME_DIR': '/invalid',
+  'PIPEWIRE_RUNTIME_DIR': meson.current_build_dir(),
   'XDG_CONFIG_HOME': meson.current_build_dir() / '.config',
   'WIREPLUMBER_CONFIG_DIR': '/invalid',
   'WIREPLUMBER_DATA_DIR': '/invalid',