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?
If base directories (e.g. $HOME/.config/fish) need to be created,
create them with mode 0700 (i.e. restricted to the owner).
This both keeps the behavior of old fish versions (e.g. 3.7.1) and is
compliant with the XDG Base Directory Specification.
See: https://specifications.freedesktop.org/basedir-spec/0.8/#referencing
