[BUGFIX] Avoid throwing away previous errors in integrity checks

With change #103465 a couple of code integrity checks based on the
`nikic/php-parser` have been combined into one file scanning script.

The goal is to collect the information per visitor type over all
files in a internal visitor state, to output per visitor result
after all php file have been scanned. However, the visitor message
state is reset for each file with the call of `startProcessing()`
method within `AbstractPhpIntegrityChecker`, thus keeping only the
result of the last error.

This change removes the message reset from the the `startProcessing()`
method to keep all error, thus displaying all errors in one go.

Resolves: #103868
Related: #103465
Releases: main
Change-Id: I96395ff648f39fed7e8216e2c32026a3071a41fe
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84333


Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>