JDK 15 RFR of JDK-8245147: Refactor and improve utility of test/langtools/tools/javac/versions/Versions.java

[Joe Darcy]

Hello,

While starting to look at the JDK 16 preparations, I noticed some 
improvements that could be made to one of the version-lated javac tests:

     JDK-8245147: Refactor and improve utility of 
test/langtools/tools/javac/versions/Versions.java
     http://cr.openjdk.java.net/~darcy/8245147.2/

This change:

     * adds non-trivial testing of new language levels
     * avoid unnecessary mixing of arrays and lists
     * refactors embedded Java sources to use text blocks
     * removes vestiges of the "1.N" naming convention
     * leaves breadcrumbs for future updates to the file

Thanks,

-Joe