Add testcase for multiple discovery-driven sources

This test was intended to reproduce the reported issue around
resources discovered for one pollster bleeding into the resource
list passed to an unrelated pollster with the same interval.

However the observed behaviour seems correct.

Related-bug: #1335986
Change-Id: Ifb608246ad2ee2d858b4268ae2e8dc6faaff025c
Merge "Add testcase for multiple discovery-driven sources"