RFR: JDK-8213185 Properly handle run-test-prebuilt -> test-prebuilt migration

Mon Feb 10 08:09:46 UTC 2020

In JDK-8213184, a rushed revert was pushed to stop breaking CI builds. A 
long-term solution is needed, where test-prebuilt can be restored to 
jib-profiles.js. This will require the run-test-prebuilt top-level make 
target in Help.gmk to be updated as well. Also, Help.gmk should really 
change name to signify that it's not just help targets, but all global 
targets (some definitely more significant!)

Bug: https://bugs.openjdk.java.net/browse/JDK-8213185
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8213185-fix-run-test-prebuilt-leftovers/webrev.01

/Magnus