Add additional LiftoverVcf tests

[bbimber]

Description

In #947 I refactored LiftOverVcf to improve performance. This just adds more test coverage, specifically around lifting symbolic and no-call alleles. I also changed a couple variables within the test DataSources to be static constants so they could be shared across tests.

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Checklist (never delete this)

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Content

• Added or modified tests to cover changes and any new functionality
• Edited the README / documentation (if applicable)
• All tests passing on Travis

Review

• Final thumbs-up from reviewer
• Rebase, squash and reword as applicable

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[coveralls]

Coverage increased (+0.01%) to 77.23% when pulling 5c66187 on bbimber:LiftoverTests into b0b9f78 on broadinstitute:master.

[bbimber]

@yfarjoun: this is related to #947. All i'm doing in this PR is adding more test coverage. I originally opened a PR for this in Nov, and just noticed other changes on master caused conflicts in my original PR. Would it be possible to get this approved and merged sooner to avoid that hassle again? I would not ask, except this is such a minor change.

[yfarjoun]

Thanks for the extra tests. and sorry for the delay.

[yfarjoun]

👍