Fix flaky test tests/test_user_invite.py::test_create_user_sends_the_invitation_by_default

The merge lane saw `tests/test_user_invite.py::test_create_user_sends_the_invitation_by_default` fail and then pass on a rerun of the same tree. - Test: `tests/test_user_invite.py::test_create_user_sends_the_invitation_by_default` - Tree: `336500b9600a16cbb307ba31a69997424d5b7820` - Lane task: 9614 - Flake record: 01a16cd3-b55c-7da4-a3bf-308fdae5e267 Make it deterministic; do not delete it unless it tests nothing. - Also flaky, same file: `tests/test_user_invite.py::test_a_new_invitees_button_creates_their_password` (tree `336500b9600a16cbb307ba31a69997424d5b7820`, lane task 9614, flake record 01a16cd3-be9b-7c50-85f1-266089e35768) - Also flaky, same file: `tests/test_user_invite.py::test_correcting_an_invitees_email_spends_the_link_the_old_address_got` (tree `336500b9600a16cbb307ba31a69997424d5b7820`, lane task 9614, flake record 01a16cd3-c32a-71ad-85c7-46b9cbcecdb3) - Also flaky, same file: `tests/test_user_invite.py::test_create_user_without_send_invitation_sends_nothing` (tree `336500b9600a16cbb307ba31a69997424d5b7820`, lane task 9614, flake record 01a16cd3-c5ef-75b0-a1b1-7fa1e2907f6b) - Also flaky, same file: `tests/test_user_invite.py::test_create_user_succeeds_whatever_the_email_does[not-configured]` (tree `336500b9600a16cbb307ba31a69997424d5b7820`, lane task 9614, flake record 01a16cd3-c8bc-7fab-8dee-ed26ffea8a98) - Also flaky, same file: `tests/test_user_invite.py::test_create_user_succeeds_whatever_the_email_does[failed]` (tree `336500b9600a16cbb307ba31a69997424d5b7820`, lane task 9614, flake record 01a16cd3-c99c-703c-806b-6d55567d1053) - Also flaky, same file: `tests/test_user_invite.py::test_resend_emails_a_user_who_has_not_signed_in` (tree `336500b9600a16cbb307ba31a69997424d5b7820`, lane task 9614, flake record 01a16cd3-ca47-72a3-a149-bcd69219fe4b) - Also flaky, same file: `tests/test_user_invite.py::test_resend_refuses_an_activated_or_disabled_user[activated_at]` (tree `336500b9600a16cbb307ba31a69997424d5b7820`, lane task 9614, flake record 01a16cd3-cb4f-7bba-be42-9c50c567d8cf) - Also flaky, same file: `tests/test_user_invite.py::test_resend_refuses_an_activated_or_disabled_user[disabled_at]` (tree `336500b9600a16cbb307ba31a69997424d5b7820`, lane task 9614, flake record 01a16cd3-cc42-716a-82ea-a26d4df161e0) - Also flaky, same file: `tests/test_user_invite.py::test_resend_without_system_mail_is_409_naming_where_to_set_it` (tree `336500b9600a16cbb307ba31a69997424d5b7820`, lane task 9614, flake record 01a16cd3-cd24-7b42-9dd5-8402518ef410)

Flake cause (receive test putting the worker's global clock at infinity) was already fixed on develop by Task 9620 (PR #6802). Dropped the duplicate commit and closed PR #6803. receive + invite tests: 52 passed on develop. Memory: none applied.