Add specs for email previews

What does this MR do and why?

This MR adds specs for preview emails.

See !97039 (comment 1091254119) for more info.

Closes #372860 (closed).

How to set up and validate locally

bin/rspec ./spec/mailers/previews_spec.rb
.....
65 examples, 0 failures, 1 pending

FOSS_ONLY=1 bin/rspec ./spec/mailers/previews_spec.rb
.....
45 examples, 0 failures, 1 pending

Draft?

• Fix two spec failures

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.