Split hspow_vectors::solver() tests?

When running tests in parallel using e.g. cargo-nextest, the test runtime depends on the runtime of the slowest test. Currently, the longest-running tests are:

        PASS [   1.180s] equix::equix_verify expected_solution_count
        PASS [   1.107s] tor-geoip test::embedded_db
        PASS [   2.282s] tor-chanmgr transport::default::test::test_connect_one
        PASS [   7.423s] tor-hspow::hspow_vectors solver

It might be worth our while to split up the solver function into separate parallelizable functions (like solver1, solver2, etc) to lower the overall runtime.

Not a super high priority.

cc @beth