Mention about other ways to run mailing in test mode

[marcincodes]

Describe the bug
Docs mention about setting NODE_ENV === "test" to enable mailing in "test mode". But in my case (nextjs) it was always defaulting to "development" even when I run the application NODE_ENV=test next dev.

To Reproduce
Steps to reproduce the behavior:

1. Run NextJS app with NODE_ENV=test
2. Mailing is not in test mode

Expected behavior
Mention in the docs you can use MAILING_INTEGRATION_TEST and TEST env variables to turn on test mode in mailing

Screenshots
If applicable, add screenshots to help explain your problem.

Desktop (please complete the following information):

• OS: [e.g. iOS]
• Browser [e.g. chrome, safari]
• Version [e.g. 22]

Smartphone (please complete the following information):

• Device: [e.g. iPhone6]
• OS: [e.g. iOS8.1]
• Browser [e.g. stock browser, safari]
• Version [e.g. 22]

Additional context
Nor bug / nor feature. Code is in place, docs are missing