GNU and BSD `mktemp` handle options differently, and it's a useful
utility for tests. As such, define a common `mktemp` function wrapper
for the test suite.
It might actually be nice to expand this for more flags and support it
globally, but that may result in confusion for any users of BSD mktemp
that expect to be running /bin/mktemp.
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.
Also tweak colored output to reset before the newline instead of after,
so travis behaves better (for some reason reset causes travis to display
the line in black).
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.
