`design_policy_spec.rb` has been flagged as a spec that we should
attempt to speed up.

It was initially running at around 20s. After these changes the spec
runs at around 7s.

Almost all of the time-saving is through the avoidance of assigning
users to roles within the `project` before each test, and instead, doing
this work once at the beginning of all tests.

https://gitlab.com/gitlab-org/gitlab/-/issues/229668