[TASK] Make usage of TYPO3_PATH_WEB failsafe

[TASK] Make usage of TYPO3_PATH_WEB failsafe

Using the TYPO3_PATH_WEB env var with a trailing
slash leads to failing unit/functional tests.

Trimming and appending a slash makes the usage
fail safe.

Releases: master
Resolves: #69124
Change-Id: I5048de4c9d35400c148cb5c40cf516d0d684b47a
Reviewed-on: http://review.typo3.org/42676
Reviewed-by: Helmut Hummel <helmut.hummel@typo3.org>
Tested-by: Helmut Hummel <helmut.hummel@typo3.org>
Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>