Running locallyInstalledAnt test method of AntPluginTest with ANT_HOME set to a version of ANT and having on PATH another version of ANT makes the test fails because the expected ANT version is get calling:

jenkins.runScript("'ant -version'.execute().text");

which probably uses ANT_HOME and then the real execution of the build task is executed using the one defined in PATH variable, and since they are different the build fails. This happens in CI environment.