Complete test coverage for query package

[stephen-soltesz]

This change restructures some logic in query package to complete test coverage there. This change moves the meat of using the bigquery package into the bqiface package. This part is not yet covered.

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 56

• 9 of 9 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+16.7%) to 100.0%

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Totals
Change from base Build 55:	16.7%
Covered Lines:	149
Relevant Lines:	149

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💛 - Coveralls

[pboothe]

Would things be implemented differently if you had known about https://github.com/googleapis/google-cloud-go-testing/tree/master/bigquery/bqiface ?

[pboothe]

Talked in person.

I agree this change is an improvement, and that i makes things better than they used to be. We both agree that this is a temporary stop on the way to a better long-term solution that it not yet clear to either of us.

[pboothe]

Reviewable status: complete! 1 of 1 LGTMs obtained