Well, I am surprised to see using Java 11 with Jython 2.7.2, but if it works…
I would experiment, to remove the .jar from the library import, and make sure the MyTestLib.jar full pass is in CLASSPATH.
Then if that does not work, I would experiment, without using .jar, but just .java and .class files.
Can’t say more, except for you to double check the User Guide.
Good luck.