javafx.scene.control.ControlTest.testRT18097 actually tests built tests classes, not the controls classes.
The reason for this is because when running "gradle test" the classpath is constructed so that the modules\controls\build\classes\test happens before the modules\controls\build\classes\main and the cl.getResource("javafx/.."); call returns modules\controls\build\classes\test when it should return modules\controls\build\classes\main.
When running tests from NB (which require some manual adding classpaths), this call returns netbeans\controls\build\classes and the test runs correctly.