Add more tests for the cluster age filter DB query

After https://github.com/deis/workflow-manager-api/pull/65 is done, the DB-level tests run through multiple filters but add a single cluster to the database on each run. Then, when the cluster is added, each run checks for its existence in the filter query's result or not.

This test should improve to add multiple clusters to the database on each iteration. Some of the clusters should be in the filter and some shouldn't, and each run should verify that the correct clusters were returned.