error about global side-effects in non-isolated tests

[staabm]

I think it would be helpful if we could error in tests which trigger a global side-effect like e.g.

• error_reporting(E_ALL) when not restored to the old value within the test, see example
• things like registration of a shutdown/autoload/tick/error-handler/exception-handler/stream-wrapper/...-function without cleanup
• changes in global/super-global variables

no error should appear in case such a pattern occurs in tests which a run using PHPUnit process isolation (@runInSeparateProcess, @runTestsInSeparateProcesses, etc)