Added new VariantContext comparison functionality for testing

[KevinCLydon]

Added the new VariantContext comparison functionality from #6417 but without yet integrating it into any existing tests.

[gatk-bot]

Travis reported job failures from build 32470
Failures in the following jobs:

Test Type	JDK	Job ID	Logs
unit	openjdk11	32470.13	logs

[KevinCLydon]

@droazen The Travis build failed so I reran it a couple times and it seems to have passed fine on the most recent run. I'm not sure if that should still be taken as an indication of an issue, though.

The first failure was on the Java 11 job, which I just assumed was the same issue other builds are running into. Second time, it was a failure on one of the Java 8 jobs and it seemed to be a docker issue, which I didn't think made sense for the type of changes in this PR.

[bbimber]

@droazen @cmnbroad @KevinCLydon This PR is blocking #6973, and it's been stuck for a while. I would like to offer my help in doing any work to get this closed out. #4571 is related and I could offer help on that and updating needed tests if this could help get all this closed out.

[bbimber]

Hello - I'm writing to check on these PRs again. I appreciate these are not GATK's priority, but they're blocking #6973 and we've been stuck for a long time here. If there is additional work, testing or anything needed on this I am happy to offer my time if there's anything that would help get this done. For example, while this PR creates the VC comparison code, it doesnt integrate it into existing tests. if doing this would help accelerate approving this I would be willing to take a stab at this.