Add unit test for target handle association (#274)
Add unit test for target handle association (#274)

Demonstrates the problem in #273.

* Test correct assignment of handles to synapse mechanisms and handles under a variety of circumstances.
halfflat/nestmc-proto,halfflat/arbor,halfflat/arbor,eth-cscs/nestmc-proto,halfflat/arbor,eth-cscs/nestmc-proto,halfflat/arbor,halfflat/nestmc-proto,eth-cscs/nestmc-proto,halfflat/nestmc-proto,halfflat/arbor