[BUGFIX] Send test email in Install Tool as HTML

Explicitly set the format of the email body to text/html.

Resolves: #60251
Releases: 6.3, 6.2
Change-Id: I350b0f4cb9e69cd20dbd439647f56cd9c641b501
Reviewed-on: https://review.typo3.org/31684
Reviewed-by: Pascal Dürsteler <pascal.duersteler@gmail.com>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>