Split `make test` into two targets `make test_low_level` and `make
test_fishscript`, primarily so fishscript tests can be rechecked quickly
after edits.
Reformat the test.fish file and update some of the code to be a little
more straightforward (e.g. `if not cmd` instead of `if cmd; else`).
This commit makes fish_parse_opt stop at first non-option argument,
which expected as fish script name and pass the remaining argument to
that script. I also updated the test cases.
