The extension is using old API message formats, these should be replaced with i18n messages with entries in en.json and qqq.json
See https://www.mediawiki.org/wiki/API:Localisation for more information. Looking at many other extensions will also provide working examples. Such as https://github.com/wikimedia/mediawiki-extensions-CodeReview/blob/master/api/ApiCodeDiff.php#L73-L95 along with https://github.com/wikimedia/mediawiki-extensions-CodeReview/blob/master/i18n/en.json#L192-L195 and https://github.com/wikimedia/mediawiki-extensions-CodeReview/blob/master/i18n/qqq.json#L218-L221
```
14:45:22 1) ApiDocumentationTest::testDocumentationExists with data set "Module configure with MiserMode=false, AllowCategorizedRecentChanges=false" ('configure', array(false, false))
14:45:22 Module description message apihelp-configure-description exists
14:45:22 Failed asserting that false is true.
14:45:22
14:45:22 /srv/jenkins-workspace/workspace/mwext-testextension-php55/src/tests/phpunit/structure/ApiDocumentationTest.php:51
14:45:22 /srv/jenkins-workspace/workspace/mwext-testextension-php55/src/tests/phpunit/structure/ApiDocumentationTest.php:80
14:45:22 /srv/jenkins-workspace/workspace/mwext-testextension-php55/src/tests/phpunit/MediaWikiTestCase.php:399
14:45:22 /srv/jenkins-workspace/workspace/mwext-testextension-php55/src/maintenance/doMaintenance.php:111
14:45:22
14:45:22 2) ApiDocumentationTest::testDocumentationExists with data set "Module configure with MiserMode=true, AllowCategorizedRecentChanges=true" ('configure', array(true, true))
14:45:22 Module description message apihelp-configure-description exists
14:45:22 Failed asserting that false is true.
14:45:22
14:45:22 /srv/jenkins-workspace/workspace/mwext-testextension-php55/src/tests/phpunit/structure/ApiDocumentationTest.php:51
14:45:22 /srv/jenkins-workspace/workspace/mwext-testextension-php55/src/tests/phpunit/structure/ApiDocumentationTest.php:80
14:45:22 /srv/jenkins-workspace/workspace/mwext-testextension-php55/src/tests/phpunit/MediaWikiTestCase.php:399
14:45:22 /srv/jenkins-workspace/workspace/mwext-testextension-php55/src/maintenance/doMaintenance.php:111
14:45:22
```