RFR: 8021601 : (xxs) Add unit test for PriorityQueue(Comparator) constructor

[Mike Duigou]

Hello all;

JDK-6799426 (http://hg.openjdk.java.net/jdk8/tl/jdk/rev/a6cbb9808e4b) was pushed without a unit test. (Always recheck for unit tests when breathing life back into a stale old patch). A unit test is needed. This change adds an additional case to an existing test to exercise the new constructor.

http://cr.openjdk.java.net/~mduigou/JDK-8021601/0/webrev/

I have also tweaked the constructor documentation per Paul Benedict's suggestion.

Thanks,

Mike