Remove dependency on dwave-tabu

The `dwave.system.testing` module requires [dwave-tabu](https://github.com/dwavesystems/dwave-tabu), see https://github.com/dwavesystems/dwave-system/blob/082c95b2d48c481a7dcf9d72d16a118a06735c68/dwave/system/testing.py#L20

This is probably an over-large dependency for a testing module. We should consider other options. To throw out an idea:
* We could use dimod's [SimulatedAnnealingSampler](https://github.com/dwavesystems/dimod/blob/d28565346867233d57bef368d668d93af39da3d9/dimod/reference/samplers/simulated_annealing.py#L36) as a fallback when `dwave-tabu` is not installed. Or even the `RandomSampler` from dimod.

See also #282