Bump test coverage for SV utils package

[SHuang-Broad]

We've been implementing new features but the coverage in the sv.utils package has been non-pretty.

Some of the classes like ExternalCommandlineProgramModule.java and GATKSVVCFConstants.java are OK (ExternalCommandlineProgramModule exists for historical reason, when we had SGA as the assembler; GATKSVVCFConstants is mostly holding String literals), but other classes are doing actual work and have low coverage.

We should test them, not via integration tests only, but also unit tests.