RFR: 8170821: Ensure access checks result in consistent answers

[Rachel Protacio]

Hello!

Please review this change adding a jtreg test to ensure that access 
check results are cached, so the same check produces the same answer 
even if the user adds access in between. (Credit to Lois for the actual 
workings of the test.)

Bug: https://bugs.openjdk.java.net/browse/JDK-8170821
Open webrev: http://cr.openjdk.java.net/~rprotacio/8170821.00/

Passes JPRT on all platforms.

Thanks,
Rachel