The tests in the \Tracker_FormElement_Field_List_Bind_UgroupsTest generate mocks based on the \UserXMLExporter class however they use an incorrect case.
As a result the class is not loaded (Composer autoloader is case sensitive) and the class end up being replaced by the mock framework. The other test classes using \UserXMLExporter might fail if they are executed after this one since they do not have access to the real class anymore, PHP is not case sensitive with the class names so the autoloading is not triggered again. This can leads to failure of the unit test suite looking like this:

1) Tracker\Creation\JiraImporter\Import\Artifact\ArtifactsXMLExporterTest::testItExportsArtifacts
Error: Call to undefined method UserXmlExporter::build()
/tuleap/plugins/tracker/tests/unit/Tracker/Creation/JiraImporter/Import/Artifact/ArtifactsXMLExporterTest.php:128

2) Tracker\Creation\JiraImporter\Import\Artifact\ArtifactsXMLExporterTest::testItExportsArtifactsPaginated
Error: Call to undefined method UserXmlExporter::build()
/tuleap/plugins/tracker/tests/unit/Tracker/Creation/JiraImporter/Import/Artifact/ArtifactsXMLExporterTest.php:128

3) Tuleap\Tracker\Artifact\Tracker_ArtifactTest::testItExportsTheArtifactToXML
Error: Call to undefined method UserXmlExporter::exportUserByUserId()
/tuleap/plugins/tracker/include/Tracker/XML/Exporter/ChangesetXMLExporter.class.php:67
/tuleap/plugins/tracker/include/Tracker/XML/Exporter/ArtifactXMLExporter.php:70
/tuleap/plugins/tracker/include/Tracker/Artifact/Tracker_Artifact.class.php:2073
/tuleap/plugins/tracker/tests/unit/Tracker/Artifact/Tracker_ArtifactTest.php:865
[...]