Email workflow test changes needed to pass in multiple browsers

[rmunn]

Describe the bug
One email workflow test I was working on was passing in Chromium but failing in Firefox, and I believe I have figured out why.

The email subject line is: "Project join request: (user name) wants to join (project name)". These are randomly-generated test users with a GUID in their name. The email is being sent to the [email protected] email address.

Currently, our email testing code in frontend/tests/email/mailbox.ts has code to open an email inbox for a certain user, and click on the email whose subject is taken from an enum, i.e. "Project join request". This works when the mailbox belongs to the generated test user, because there will be only one "Verify Email" message in the inbox. But when the mailbox belongs to [email protected], the username being searched for needs to be part of the subject line searched, so we'll need to allow arbitrary strings to be passed in to the mailbox.ts openEmail function.

I'm recording this as a separate issue because I might not get to it right away, but I want to keep track of my findings.