Fix failing test in spec/workers/post_receive_spec.rb

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