test all solvers on the system, allow user to change default solver

1. `get_default_solver` and `set_default_solver` can be used to retrieve the current default solver.
2.  `can_solve_mip` and `can_solve_sdp` can be used to see if a solver supports those operations. So far Mosek, Gurobi and GLPK support MIPs and only SCS supports SDPs/ exponential cones.
3. We can use `include test/run_tests_all_solvers.jl` to run every test a solver supports.
4. We can use `include test/run_tests.jl` to run all the tests supported by the current default solver.
5. Including a test file directly does not check to see if the solver supports those cones.