tests: Tweak install tests to use flatpak-repo instead of install

Also drop the stray `--user` argument to flatpak-builder

Attempt to fix the ocassional flakiness in CI due to random timeouts
[1] [2] by avoiding installing.

[1]: https://github.com/flatpak/flatpak-builder/actions/runs/22909321693/job/66476906753#step:7:1585
[2]: https://github.com/flatpak/flatpak-builder/actions/runs/22905637946/job/66463413480#step:7:1583

Author: bbhtt

tests/test-builder.sh

@@ -176,11 +176,11 @@ else
 fi
 
 # test install
-${FLATPAK_BUILDER} --user --install \
+${FLATPAK_BUILDER} --repo=$REPO/repo_test_install \
     --force-clean builddir org.flatpak.install_test.json >&2
-REFS=$(flatpak list --all --columns=ref 2>/dev/null)
-echo "$REFS" | grep -q "org\.flatpak\.install_test"
-echo "$REFS" | grep -q "org\.flatpak\.install_test\.Debug"
-echo "$REFS" | grep -q "org\.flatpak\.install_test\.Locale"
+REFS=$(flatpak repo --branches $REPO/repo_test_install 2>/dev/null)
+echo "$REFS" | grep -q "app/org\.flatpak\.install_test/"
+echo "$REFS" | grep -q "runtime/org\.flatpak\.install_test\.Debug/"
+echo "$REFS" | grep -q "runtime/org\.flatpak\.install_test\.Locale/"
 
 echo "ok install"