[E2E] Adding additional check to custom_email if value is updated

What does this MR do and why?

Fixes the failure https://gitlab.com/gitlab-org/quality/test-failure-issues/-/issues/14571, which seems to be flaky. From the job logs https://ops.gitlab.net/gitlab-org/quality/staging-canary/-/jobs/21418825 it looks like the previous text is checked against the new text. So adding additional validation so that it waits until the new value gets updated.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

cd qa

bundle exec rspec qa/specs/features/ee/browser_ui/2_plan/custom_email/custom_email_spec.rb

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