This is somewhat subtle:
The #RUN line in a littlecheck file will be run by a posix shell,
which means the substitutions will also be mangled by it.
Now, we *have* shell-quoted them, but unfortunately what we need is to
quote them for inside a pre-existing layer of quotes, e.g.
# RUN: fish -C 'set -g fish %fish'
here, %fish can't be replaced with `'path with spaces/fish'`, because
that ends up as
# RUN: fish -C 'set -g fish 'path with spaces/fish''
which is just broken.
So instead, we pass it as a variable to that fish:
# RUN: fish=%fish fish...
In addition, we need to not mangle the arguments in our test_driver.
For that, because we insist on posix shell, which has only one array,
and we source a file, we *need* to stop having that file use
arguments.
Which is okay - test_env.sh could previously be used to start a test,
and now it no longer can because that is test_*driver*.sh's job.
For the interactive tests, it's slightly different:
pexpect.spawn(foo) is sensitive to shell metacharacters like space.
So we shell-quote it.
But if you pass any args to pexpect.spawn, it no longer uses a shell,
and so we cannot shell-quote it.
There could be a better way to fix this?
fish might use XDG_RUNTIME_DIR for the uvar notifier fifo, so this
makes sure that tests are isolated.
Also set permissions to comply with the XDG basedir spec.
This lets us run non-fish targets (such as `fish_tests`) under a clean
test environment without running into the fish-specific payload
configuration now carried out by `test_driver.sh` which expects a
`.fish` payload that it will run under a deterministically configured
instance of fish, running in an environment initialized by
`test_env.sh`.
This should fix the problem with in-tree builds leaving detritus behind
after a `make test` when `fish_tests` would be executed without
`test_driver.sh` - it is now executed under `test_env.sh` instead.
