Bug 21817: Centralize the mock of userenv from tests

Bug 21817: Centralize the mock of userenv from tests

This patch adds a new method mock_userenv from t::lib::Mocks in order to
simplify the mock of the userenv.

Test plan:
prove all the test files modified by this patch

Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>

Signed-off-by: Nick Clemens <nick@bywatersolutions.com>
(cherry picked from commit ca124b5c14a3f0a34fafe307fec9ade4008eaa41)
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>