The extension is using old API message formats, these should be replaced with i18n messages with entries in en.json and qqq.json
As well as message apihelp-linkfilter-description, descriptions for the 2 parameters, and one for the example is necessary
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
19:07:52 1) ApiDocumentationTest::testDocumentationExists with data set "Module linkfilter with MiserMode=false, AllowCategorizedRecentChanges=false" ('linkfilter', array(false, false))
19:07:52 Module description message apihelp-linkfilter-description exists
19:07:52 Failed asserting that false is true.
19:07:52
19:07:52 /srv/jenkins-workspace/workspace/mwext-testextension-php55/src/tests/phpunit/structure/ApiDocumentationTest.php:51
19:07:52 /srv/jenkins-workspace/workspace/mwext-testextension-php55/src/tests/phpunit/structure/ApiDocumentationTest.php:80
19:07:52 /srv/jenkins-workspace/workspace/mwext-testextension-php55/src/tests/phpunit/MediaWikiTestCase.php:399
19:07:52 /srv/jenkins-workspace/workspace/mwext-testextension-php55/src/maintenance/doMaintenance.php:111
19:07:52
19:07:52 2) ApiDocumentationTest::testDocumentationExists with data set "Module linkfilter with MiserMode=true, AllowCategorizedRecentChanges=true" ('linkfilter', array(true, true))
19:07:52 Module description message apihelp-linkfilter-description exists
19:07:52 Failed asserting that false is true.
19:07:52
19:07:52 /srv/jenkins-workspace/workspace/mwext-testextension-php55/src/tests/phpunit/structure/ApiDocumentationTest.php:51
19:07:52 /srv/jenkins-workspace/workspace/mwext-testextension-php55/src/tests/phpunit/structure/ApiDocumentationTest.php:80
19:07:52 /srv/jenkins-workspace/workspace/mwext-testextension-php55/src/tests/phpunit/MediaWikiTestCase.php:399
19:07:52 /srv/jenkins-workspace/workspace/mwext-testextension-php55/src/maintenance/doMaintenance.php:111
19:07:52