Don't count qpsolvers matrix build times in solver times

[stephane-caron]

Currently we time solvers externally, so that the timing will include both matrix build and solve times. This is done in qpbenchmark/utils.py by, essentially:

start_time = perf_counter()
solution = qpsolvers.solve_problem(problem, solver=solver)
runtime = perf_counter() - start_time

Separating build and solve times would allow a more fair comparison. It should be done upstream in qpsolvers: qpsolvers/qpsolvers#255