[BUGFIX] Allow sending test mails via pressing enter key

The form for sending test mails in the Install Tool now listens to the
`submit` event which allows to trigger the submit via pressing the
"enter" key.

Resolves: #90312
Releases: master, 9.5
Change-Id: Id37e285702d2bb8abc790d8b092598cf3201b8aa
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/63539


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Christian Eßl <indy.essl@gmail.com>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Christian Eßl <indy.essl@gmail.com>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>