fix: FeatureTest may cause risky tests

[kenjis]

Description
See codeigniter4/shield#802

1) Tests\Authentication\Filters\PermissionFilterTest::testFilterNotAuthorized
Test code or tested code did not (only) close its own output buffers

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide

[kenjis]

@datamweb Thank you for the review!