Fix $pid -> $fish_pid in signals.expect

This was a expect variable, not a fish one.

tests/signals.expect

@@ -12,13 +12,13 @@ expect_prompt
 send "sleep 131 &\r"
 expect_prompt
 send "sleep 132\r"
-exec -- kill -HUP $fish_pid
+exec -- kill -HUP $pid
 
 # Verify the spawned fish shell has exited.
 catch {expect default exp_continue} output
 wait
 
-# Verify all child processes have been killed. We don't use `-p $fish_pid` because
+# Verify all child processes have been killed. We don't use `-p $pid` because
 # if the shell has a bug the child processes might have been reparented to pid
 # 1 rather than killed.
 set status [catch {exec pgrep -l -f "sleep 13"} output]