JDK 12 RFR of JDK-8206114: Refactor langtools/tools/javac/classfiles/ClassVersionChecker.java

[joe darcy]

Hello,

Another day, another version-related test refactoring:

     JDK-8206114: Refactor 
langtools/tools/javac/classfiles/ClassVersionChecker.java
     http://cr.openjdk.java.net/~darcy/8206114.0/

I've verified the ease of maintenance of the test by removing the TWELVE 
enum value and running the test against JDK 11.

This is overlap in the test coverage of this test and other other test 
out for review, but I haven't not tried to precisely determine or 
eliminate that overlap.

Thanks,

-Joe