[foreign-abi] RFR 8230806: Use libTestUpcall in TestUpcall

[Jorn Vernee]

Hi,

While working on another patch I noticed the current TestUpcall test is 
loading the downcall test library instead of the upcall test library. 
This means that the upcall tests were not actually using the callback 
that they were being passed. I've changed the library that is loaded, 
and added some checks that check that the callback is actually being 
called. (this also revealed a small bug in UniversalUpcallHandler that I 
fixed)

Please review the following:

Bug: https://bugs.openjdk.java.net/browse/JDK-8230806
Webrev: 
http://cr.openjdk.java.net/~jvernee/panama/webrevs/8230806/webrev.00/

Thanks,
Jorn