Originally suggested on IRC.

https://test.musicbrainz.org/ currently shows the following banner:

This is a MusicBrainz development server. Return to musicbrainz.org.

It should provide more details about it such as:

• How the sandbox mode works: The data can be edited without risk for the main database.
• The date of the data dump that has been used.
• Possibly a link to the git commit being currently tested (even though it is already mentioned from the footer).

These details were provided in an admin-editable banner but it has been lost since it can be edited by logged-in users for test.