Don't try to execute generator tests if they do not exist

[alexanderpann]

When the devkits for generator tests are not included, generator tests will fail with a ModuleClassNotFoundException when executing the test. We could generate a dummy test that gets ignored or automatically fall back to the interpreter test here: http://127.0.0.1:63320/node?ref=r%3A7961970e-5737-42e2-b144-9bef3ad8d077%28org.iets3.core.expr.tests.behavior%29%2F7985877191952440064