Update the test runners so they set up their own environment in
test_util.fish. This simplifies the Makefile and paves the way for
adding utility functions for use in the tests themselves.
Split test_interactive off from test_fishscript and add a new target
test_high_level that tests both.
Add some Makefile magic so the tests can be run serially without using
sub-make, which gets rid of a little noise from the make output.
Rewrite interactive tests to look better.
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.
