Inconsistences in "test" vs "tests" core API / subsystem

As per https://docs.moodle.org/dev/Core_APIs we have "test" core API and as such, it can be used as a 2nd level in namespaces.

There is tool_policy\test namespace used in ./admin/tool/policy/classes/test/helper.php

But then, I am seeing many others using "tests", e.g. privacy/classes/tests/ or /message/classes/tests/ or tag/classes/tests/ or more recently:

namespace enrol_oneroster\tests\fixtures\local;

Additionally, should not this subsystem / API be also enlisted explicitly in lib/components.json ?

TIA for clarification for us good citizens who wish to obey the rules.