[FIX] mail:tests: no undeterministic failure of discuss history test

Before this commit, the test 'all messages in "Inbox" in "History"
after marked all as read' had non-deterministic failures.

More precisely, it crashed on either assertion "there should be 30
messages in History" or "there should be 40 messages in History",
both resulting to 0 messages in History instead.

Unfortunately, as of the date of this commit, we still do not
understand why it rarely fails. In the meantime, we have decided to
make these errors less likely to happen, by waiting much longer
before the assertions. We are aware that this is a poor solution,
but this is much better than skipping the test, until we find and fix
this issue at a later moment.

Closes #43072

Signed-off-by: Sébastien Theys (seb) <seb@odoo.com>