[IMP] test_lint: add a test to detect git conflict markers

Since the deployement of the forward-port bot, chances to merge code
with conflict markers has greatly increased.

With this commit, a new test is added to grep for those markers in most
common code files.

closes odoo/odoo#39813

Signed-off-by: Christophe Monniez (moc) <moc@odoo.com>