Suppress e-mails on failed builds if allow_failure is set

Every time I push to GitLab, I get > 2 emails saying a spec failed when I don't care about the benchmarks and others that have `allow_failure` set to `true`.

@ayufan mentioned creating a summary e-mail to prevent getting one e-mail per build, but the latter might actually be desirable. For example, I do want to know if Rubocop errors fail right away.

See merge request !2178