April 2021, this is outdated and needs reviewing. MD
Based on the Travis-CI testing of phpList itself, https://travis-ci.org/phpList/phplist3, a plugin can be tested as well.
This is done by defining the Git repo of phpList as a dependency in composer. Once composer checks out phpList (without UI or anything, but for functionality that is not a problem) the config file can set the Plugin rootdir and you can run your unit tests.
Travis will test every commit and send you the results