[DOC] test_mail: update performance counters

Commit 42659dc7 recently done in saas-11.2 added performance test cases
in test_mail module allowing to cover more use cases. Since release of
saas-11.2 some optimizations already landed in master pre-saas-11.3 and
some features have been refactored.

This implies updating the performance counters. This is done in a separate
commit to distinguish previous-counters from current counters instead of
loosing it in a diff.

Most message-post related tests have been improved by some queries as
message creation has been speeded up at f727e9d9. Some other commits
like 9de5d417 also slightly improved performances. Activities-related
performance have been improved at e8998123. Tests case involving
assignation notification perform worst than in saas-11.1 . It is linked to
the feature using more advanced email and templates as well as working for
Inbox too which is not the case in saas-11.2. This improvement has been done
at e9734f49 .

This commit is related to task 1824965. Thanks to @KangOl for being
nice with people trying to make its forward-port more difficult.