Fixed: Suppressed ErrorExceptions when running PHPUnit with `processIsolation="true"`

In #33 it was discovered that when running PHPUnit with `processIsolation="true"` an [error handler is set](https://github.com/sebastianbergmann/phpunit/blob/master/src/Util/PHP.php#L138-L140) which throws `\ErrorException`s for any userland warnings/errors.

In this case errors should be suppressed.