tests: Add TEST_SKIP_CLEANUP env var for skipping test cleanup

This mirrors the same environment variable in OSTree’s unit tests, which
keeps the temporary directory around after tests have completed (or
failed) so the developer can examine it.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

tests/libtest.sh

@@ -295,6 +295,10 @@ cleanup () {
     /bin/kill $DBUS_SESSION_BUS_PID ${FLATPAK_HTTP_PID:-}
     gpg-connect-agent --homedir "${FL_GPG_HOMEDIR}" killagent /bye || true
     fusermount -u $XDG_RUNTIME_DIR/doc || :
-    rm -rf $TEST_DATA_DIR
+    if test -n "${TEST_SKIP_CLEANUP:-}"; then
+        echo "Skipping cleanup of ${TEST_DATA_DIR}"
+    else
+        rm -rf $TEST_DATA_DIR
+    fi
 }
 trap cleanup EXIT