Refactor gnitests to scale when NUMEPS is changed

[e-harvey]

Right now, each ep is in a separate domain but many assumptions are made throughout the tests about what a given ep should be used for.

For example, in many tests ep[0] refers to the sender's end point and ep[1] to the remote peer's. For these tests, it would be better to use self documenting names and have two different variables, send_ep and recv_ep.

For tests that use a given ep for sending and receiving these tests should be modified to scale when NUMEPS is increased or decreased to as low as one.