*/rtstubs.jar are copied from http://git.eclipse.org/c/jdt/eclipse.jdt.ui.git/tree/org.eclipse.jdt.ui.tests/testresources/rtstubs*.jar

rtstubs11.jar doesn't exist upstream so for now it's a copy of rtstubs10.jar, just so that we can check proper Java 11 configuration is supported. 
We obviously can't check the new APIs with it. It should be replaced by the real deal as soon as rtstubs11.jar is available upstream.