[FIX] test_themes: prepare restoring the website switcher

The forward-port of [1] made no adaptation to the code, and it now
simply crashes when trying to use the website switcher when test_themes
is installed.

This commit, joined to another related fix in community, will be adapted
in the forward-ported version to actually fix the issue. Meanwhile, it
fixes a linter error introduced by that same [1] commit.

[1]: c3af5d9

closes #1030

Related: odoo/odoo#190877
Signed-off-by: Quentin Smetz (qsm) <[email protected]>

Author: qsm-odoo