Test case for initctl signal

This commit implements a test for the signal feature of initctl. The
existing service used in tests (`common/service.sh`) has been extended
with a signal trap handler. When SIGUSR1 is received, it will write the
string `'USR1'` to `/tmp/usr1.log`. The test will run `initctl signal
service.sh SIGUSR1` and assert that the contents of the file
`/tmp/usr1.log` really is `USR1`.

test/common/service.sh

@@ -2,6 +2,10 @@
 
 set -eu
 
+# Hook SIGUSR1 and dump trace to file system
+# shellcheck disable=SC2172
+trap 'echo USR1 > /tmp/usr1.log' 10
+
 echo $$ > /run/service.pid
 
 while true; do