Changed the physics test, which was flawed. The reason is as follows.
With only one body, there can be no gravity, and even if one sets the
acceleration to a non-zero value, it gets overwritten (rightly) by 0
during the computation of gravity interactions. As such, the body stays
put. Instead, I have added a test with a non-zero constant velocity, and
zero acceleration (the value of acceleration is immaterial, in fact, I
will add a test like that in the next commit to ensure this invariant).
This corresponds to motion on a line, and this is tested.