Complete coverage of Alertmanager packages

[stephen-soltesz]

This change completes unit test coverage of the alertmanager packages.

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This change is 

[coveralls]

Pull Request Test Coverage Report for Build 104

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+9.3%) to 100.0%

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Totals
Change from base Build 97:	9.3%
Covered Lines:	290
Relevant Lines:	290

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💛 - Coveralls