Mock unnecessary calls to conductor_api.build_instances()

Some tests make unnecessary calls to conductor_api.build_instances()
when the tests themself do not depend on instances being built.

This in turn results in threads completely out of control of the tests
that spawned them that generate random errors in logs.

Change-Id: I983f9477efc9c559e3df405c590df31683eef3d7
Partial-Bug: #1371587