Add basic signal test

src/signal.cpp

@@ -273,6 +273,7 @@ static void handle_sigalarm(int sig, siginfo_t *info, void *context) {
     UNUSED(info);
     UNUSED(context);
     if (reraise_if_forked_child(sig)) return;
+    default_handler(sig, info, context);
 }
 
 void signal_reset_handlers() {

tests/signal.in

@@ -0,0 +1,6 @@
+function alarm --on-signal ALRM
+  echo ALRM received
+end
+
+kill -s ALRM $fish_pid
+

tests/signal.out

@@ -0,0 +1 @@
+ALRM received