This replaces the test_driver.sh/test.fish/interactive.fish system with a test driver written in python that calls into littlecheck directly and runs pexpect in a subprocess.
This means we reduce the reliance on the fish that we're testing, and we remove a posix sh script that is a weird stumbling block (see my recent quest to make it work on directories with spaces).
To run specific tests, e.g. all the tmux tests and bind.py:
tests/test_driver.py target/release/ tests/checks/tmux*.fish tests/pexpects/bind.py
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