Stop the runtime in PhysicsManagerTests, AddRemoveConstraint.

This test adds constraints to the PhysicsManager, then removes them.
Sporadically, the PhysicsManager will update after the add and before
the removal, which causes an assertion because no representations have
been added to the PhysicsManager.  This commit stops the runtime to
prevent any updates from occurring.
PhysicsManagerTests only starts runtime for InitTest.

Other tests should explicitly start the runtime and ensure an
update if they want to test the update code for some situation.

Otherwise (if the runtime is always running), a test may inadvertently
put the PhysicsManager in a situation that causes an assert
sporadically based on when the update occurs.
Cleanup: PhysicsManager unit tests only need a Runtime for InitTest.
Merge branch 'fix/physicsmanagertest-addremoveconstraint' into 'master'

Fix sporadic assert in PhysicsManagerTests, by stopping runtime.

+1: Julien Lenoir, kelliher, Gughan Sathyaseelan

Merged-on: https://www.assembla.com/code/OpenSurgSim/git/merge_requests/1020964