TempIOC can be used as a context manager to make PVs accessible
via EPICS during tests. IOCFactory uses Entry trees to define and
instantiate TempIOCs, so that tests can choose which PVs they
need to be available.

Since ControlLayer sets its .shims to SHIMS by default, setting
attributes in dummy cl .shims changes SHIMS for the rest of the
test session. Creating a new dict for the dummy_shim fixture avoids
this side-effect.

For redundancy, I also changed ControlLayer.__init__ to copy SHIMS
instead of using the global instance.