This finds the first broken component, to help people figure out where
they misspelt something.
E.g.
```
echo foo >/usr/lob/systemd/system/machines.target.wants/var-lib-machines.mount
```
will now show:
```
warning: Path '/usr/lob' does not exist
```
which would help with seeing that it should be "/usr/lib".
Tests are now executed in a test-specific temporary directory, so test
output on failure should be reproducible/reusable as-is without needing
to have TMPDIR defined (as it only exists by default under macOS).
Aside from the fact that the shared state could cause problems, tests
were randomly assuming it would be created where that wasn't the case.
In particular, `redirect.fish` and `basic.fish` were failing on only
macOS because `../test/temp` didn't exist yet - it would be created by
other tests later.
The user may write for example:
echo foo >&5
and fish would try to output to file descriptor 5, within the fish process
itself. This has unpredictable effects and isn't useful. Make this an
error.
Note that the reverse is "allowed" but ignored:
echo foo 5>&1
this conceptually dup2s stdout to fd 5, but since no builtin writes to fd
5 we ignore it.
Prior to this fix, if stdin were explicitly closed, then builtins would
silently fail. For example:
count <&-
would just fail with status 1. Remove this limitation and allow each
builtin to handle a closed stdin how it sees fit.
Appending to an fd doesn't really make sense, but we allowed the
syntax previously and it was actually used.
It's not too harmful to allow it, so let's just do that again.
For the record: Zsh also allows it, bash doesn't.
Fixes#6614
This was previously required so that, if there was a redirection to a
file, we would fork a process to create the file even if there was no
output. For example `echo -n >/tmp/file.txt` would have to create
file.txt even though it would be empty.
However now we open the file before fork, so we no longer need special
logic around this.
