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?
See the changelog additions for user-visible changes.
Since we enable/disable terminal protocols whenever we pass terminal ownership,
tests can no longer run in parallel on the same terminal.
For the same reason, readline shortcuts in the gdb REPL will not work anymore.
As a remedy, use gdbserver, or lobby for CSI u support in libreadline.
Add sleep to some tests, otherwise they fall (both in CI and locally).
There are two weird failures on FreeBSD remaining, disable them for now
https://github.com/fish-shell/fish-shell/pull/10359/checks?check_run_id=23330096362
Design and implementation borrows heavily from Kakoune.
In future, we should try to implement more of the kitty progressive
enhancements.
Closes#10359
Changes it from
```
$fish_color_user: not set in local scope
$fish_color_user: set in global scope, unexported, with 1 elements
$fish_color_user[1]: length=3 value=|080|
$fish_color_user: set in universal scope, unexported, with 1 elements
$fish_color_user[1]: length=7 value=|brgreen|
```
(with the trailing empty line - not just a newline)
to
```
$fish_color_user: set in global scope, unexported, with 1 elements
$fish_color_user[1]: |080|
$fish_color_user: set in universal scope, unexported, with 1 elements
$fish_color_user[1]: |brgreen|
```
This one tests a bunch of separate stuff, so we put it into a few
different files.
The main, new one is "slices.fish", which tests various index expressions.
