This makes test_low_level, test_interactive, test_invocation, and
test_fishscript independent. This allows running a smaller subset of tests.
To prevent all tests running in parallel, we also have new targets
serial_test_low_level, serial_test_interactive, etc. which have the
dependency chain that enforces serial execution.
This adds support for .check files inside the tests directory. .check
files are tests designed to be run with littlecheck.
Port printf test to littlecheck and remove the printf.in test.
In tests we would like to arrange for an executable to invoke certain
system calls, e.g. to claim or relinquish control of the terminal. This is
annoying to do portably via e.g. perl. fish_test_helper is a little
program where we can add custom commands to make it act in certain ways.
