Updating `test_sequtils.py`.

Correcting test and adding new test for `get_longest_reading_frames`
function of `sequtils` module.

`get_longest_reading_frames` is currently passing tests.
Updating `reverse_complement_to_longest_reading_frame`.

Updating `reverse_complement_to_longest_reading_frame` function of the
`seqmod` module. It now uses `get_longest_reading_frames` function to
determine the returned orientation for each sequence.

Updating tests in `test_seqmod.py` accordingly, and they are passing.