Included MAIL_FROM_ADDR in phpunit configuration

[marcusmoore]

Description

In #15839 I asked @Godmartinz to add MAIL_FROM_ADDR to .env.testing in order to get tests back to green. This worked but I really should have suggested adding it to phpunit.xml so future devs wouldn't have to notice the addition of the variable to .env.testing.example and add it locally themselves.

This PR does handles this better by included it in phpunit.xml.

Type of change

• Chore?

[what-the-diff]

PR Summary

• Removal of MAIL_FROM_ADDR from a Config File
  We have removed the email configuration MAIL_FROM_ADDR from a specific config file, .env.testing.example.
• Introduction of MAIL_FROM_ADDR for Testing Environment
  We introduced the same email configuration in another file phpunit.xml. This change allows us to use a predetermined email address (app@example.com) during software testing, streamlining the testing process.