Refactor peergrouper tests to avoid cycle imports

The tests for peergrouper, being part of peergrouper package
would trigger an import cycle if anything under apiserver
imports peergrouper.
The tests where refactored to use the export_test pattern
so all tests are under peergrouper_test package
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Addressed comments by @fwereade
Merge pull request #943 from perrito666/refactor_peergrouper_tests

Refactor peergrouper tests to avoid cycle imports

The tests for peergrouper, being part of peergrouper package
would trigger an import cycle if anything under apiserver
imports peergrouper.
The tests where refactored to use the export_test pattern
so all tests are under peergrouper_test package