Otherwise it would print "Unknown Signal" on Linux. I didn't see an
obvious way to check signal validity, plus it hardly matters.
Also mimic the output from BSD strsignal on Linux.
I kinda hate how fussy clang-format is. It reflows text
constantly (line limit), forces things onto one line *except* when
they're too long, and wants to turn this:
```c++
return true;;
```
into this:
```c++
return true;
;
```
instead of, you know, eliminating the second semicolon?
Anyway, it is what it is and we use it, I'll just look into getting some
more slack.
'fish_test_helper print_pid_then_sleep' tried to sleep for .5 seconds,
but instead it divided by .5 so it actually slept for 2 seconds.
This exceeds the maximum value on NetBSD so it wasn't sleeping at all
there.
Fixes#6476
The IO cleanup left file redirections open in the child. For example,
/bin/cmd < file.txt would redirect stdin but also leave the file open.
Ensure these get closed properly.
In tests we would like to arrange for an executable to invoke certain
system calls, e.g. to claim or relinquish control of the terminal. This is
annoying to do portably via e.g. perl. fish_test_helper is a little
program where we can add custom commands to make it act in certain ways.
