QA: Allow the registration e2e test to be skipped
What does this MR do?
Allows tests to be skipped if they require registration.
This is needed because registration is disabled on Staging and so the registration test fails every night (hence ~"Pick into 11.4").
gitlab-qa!183 (merged) adds support for the new
SIGNUP_DISABLED env var to
gitlab-qa so will need to be merged at the same time.