A test case string translation of "AM" and "PM" (as part of a Time) triggered a downstream exception in https://phabricator.wikimedia.org/T363215, thus upstreaming this from https://phabricator.wikimedia.org/T363364. See also D25618 for the specific issue.
private function loadLibraryFiles() in https://we.phorge.it/source/phorge/browse/master/src/infrastructure/internationalization/management/PhabricatorInternationalizationManagementExtractWorkflow.php$279 was added in https://we.phorge.it/rPccc7c1b42436dcdbc19d1239427f31cc6a02c7ba.
It makes sense not to make the upstream core code extract strings in test files for localization in order not to waste translators' time.