BLXV-83 (8.36) Add a PvManagerWithMockCAClients for testing.

This allows you to pass a PvManagerWithMockCAClients into a class you
want to test in place of a real PvManager. This implements all of the
CAClients as assertable Mock objects.

Also add a Python standard __repr__.
Merge remote-tracking branch 'i15-test/8.36' into 8.36