[FIX] base: correct unit test for compatibility with PIL 7.0

The unit test in `test_base_document_layout` does not pass with
version 7.0.0 of PIL, which is loaded by default on Ubuntu 20.04.
The reason is that the color obtained from the image is slightly
different.  This fix slightly relaxes the test (in fact we don't care
about the exact color obtained in this test, we just want to make sure
that it doesn't crash)

opw-2342394 (original ticket)

closes odoo/odoo#59856

Signed-off-by: Thomas Dieuzeide <tdi-odoo@users.noreply.github.com>