CRM-12922 - Allow unit-tests (e.g. api_v3_HRJobTest) to override _populateDB()

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* CRM-12922: Create data model for Job Position, etal
  http://issues.civicrm.org/jira/browse/CRM-12922
Merge pull request #1126 from totten/master-civiunittest-populatedb

CRM-12922 - Allow unit-tests (e.g. api_v3_HRJobTest) to override _populateDB()