JDK 16 RFR of JDK-8246222: Rename javac test T6395981.java to be more informative

[Joe Darcy]

Hello,

One of the langtools tests which gets updated on each release is 
T6395981.java and I'd like to give it the more descriptive name 
TestGetSourceVersions.java. Please review the webrev for this:

     http://cr.openjdk.java.net/~darcy/8246222.0/

Thanks,

-Joe