ci: Install tests in a separate prefix

Avoid using `sudo` in the installed-tests CI job, since we're installing
Meson as the current user.

.gitlab-ci.yml

@@ -150,20 +150,28 @@ installed-tests:
   stage: build
   needs: []
   variables:
-    EXTRA_MESON_FLAGS: "--prefix=/usr --libdir=/usr/lib64 -Dinstall-tests=true"
+    EXTRA_MESON_FLAGS: "-Dinstall-tests=true"
     G_TEST_ACCESSIBLE: 1
   script:
     - .gitlab-ci/show-info-linux.sh
     - export PATH="$HOME/.local/bin:$PATH"
     - pip3 install --user meson~=0.64
     - meson subprojects download
-    - meson setup ${COMMON_MESON_FLAGS} ${EXTRA_MESON_FLAGS} ${BACKEND_FLAGS} ${FEATURE_FLAGS} _build
+    - meson setup
+        --prefix="${CI_PROJECT_DIR}/_installed"
+        ${COMMON_MESON_FLAGS}
+        ${EXTRA_MESON_FLAGS}
+        ${BACKEND_FLAGS}
+        ${FEATURE_FLAGS}
+        _build
     - meson compile -C _build
-    - sudo meson install -C _build
+    - meson install -C _build
+    - export LD_LIBRARY_PATH=${CI_PROJECT_DIR}/_install/lib64
     - dbus-run-session xvfb-run -a -s "-screen 0 1024x768x24"
             gnome-desktop-testing-runner
                    --report-directory=_build/installed-tests-report/failed/
                    --parallel=0
+                   --dirs "${CI_PROJECT_DIR}/_installed/share"
                    gtk-4.0
   artifacts:
     paths: