tests/checks/disown: fix signal delivery race

Intermittent test failure suggests that kill(3p) returns before the
signal is delivered.  Fix the failure by waiting until the signal
has been delivered before continuing the test.

Fixes #12635

tests/checks/disown.fish

@@ -14,6 +14,9 @@ status job-control full
 sleep 1 &
 set -l pid (jobs -lp)
 kill -SIGSTOP $pid
+while not jobs | grep -q stopped
+    sleep .01
+end
 disown
 # CHECKERR: disown: job 1 ('sleep 1 &') was stopped and has been signalled to continue.
 echo $status