[TASK] More strict phpunit testing

[lolli42]

• Activate a couple of "fail more often" flags, and/or make them more explicit.
• Drop invalid coverage tag - it does not exist and phpunit mumbles with "Element 'coverage': This element is not expected."
• error_reporting default is E_ALL & ~E_DEPRECATED & ~E_STRICT. In general, we should strive for E_ALL error free tests: This helps to find issues with younger PHP versions more quickly.

[helhum]

Thanks!