Change several `hpat.jit` to `numba.njit` decorators in regular test system

Could you please chenge several `hpat.jit` to `numba.njit` (please use njit with default paralelization)  dicorators in our regular test system? For example, in `test_series.py` with several tests which are already support `new style` functionality?

_Originally posted by @shssf in https://github.com/IntelPython/hpat/pull/253_