The Java syystem (boot) classloader is inconsistent w.r.t. empty entries on the bootclasspath. They are ignored for the purposes of loading classes, but not ignored when locating resources. The latter behavior affects this test which uses getResource() to locate a class file. In the specific circumstances of this test (which is in the unnamed package, per usual), the empty path element can cause getResource to return an unexpected result.
The fix/workaround is to use System.getProperties("test.classes") instead of getResource().