Move slow regressions and update guidelines. (#6508)

Move slow regressions and update guidelines. (#6508)

This moves regression test that exceed the time limit of their
respective level to the appropriate level. It further updates the
guidelines in the README with information on how to properly categorize
regression tests into levels (with time limits).

Note: Test regress3/issue4717.smt2 was previously unsolved (unknown) and
      is now sat (Z3 agrees).