Make error level of tests configurabe for most tests
In many cases I'd like to have configurable error levels.
- for the Source Integrity Test since in my eyes not every core modification is an error
- file checks: I check for ENABLE_INSTALL_TOOL and I think a warning would be sufficent
- TYPO3 version check: if you want to check for latest core (not insecure core)
So please add the possibility to set the error level in most tests and leave it to the user to decide whether he wants a warning or an error.