Added test of average_gate_fidelity.
Added test of average_gate_fidelity.

qutip/qutip,zasdfgbnm/qutip,cgranade/qutip,anubhavvardhan/qutip,zasdfgbnm/qutip,cgranade/qutip,anubhavvardhan/qutip,qutip/qutip