This is what was happening before:

1. `Project#set_timestamps_for_create` was called at creation time
and set the `last_activity_at` and `last_repository_updated_at`
to the current timestamp T.

2. The test ran `PostReceive#perform`, which then called
`PostReceive#process_wiki_changes`. If less than 500 milliseconds
elapsed since T, then the update would just set the timestamp to T.

To fix this problem, we can just use Timecop to ensure at least
one second has elapsed after attempting to process changes.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/8871