tests: make sure sway's subprocesses are killed before killing sway

This fixes the tests when run within Debian's autopkgtest. Since there
were still some living child processes, autopkgtest was stuck waiting
for them forever.

tests/utils/mod.rs

@@ -107,6 +107,10 @@ impl Sway {
     }
 
     fn stop(&mut self) {
+        // in case some apps were spawned, kill them, and give them some time to be killed
+        self.send_command(["[all] kill"].as_slice());
+        std::thread::sleep(std::time::Duration::from_millis(500));
+        // now terminate sway
         signal(self.process.id(), "TERM");
         match self.check_connection("Unable to connect to") {
             Ok(()) => eprintln!("Sway terminated correctly on its own"),