Optimize notification emails specs

What does this MR do?

This MR optimizes some specs for notification emails.

Before:

Finished in 1 minute 40.79 seconds (files took 0.94382 seconds to load)
368 examples, 0 failures, 358 projects created

After:

Finished in 48.83 seconds (files took 0.94741 seconds to load)
362 examples, 0 failures, 55 projects created

Almost one minute better, locally.

What are the relevant issue numbers?

See https://gitlab.com/gitlab-org/gitlab-ce/issues/37234