unit-tests: Make TLS echo server cancelable

Seems to be required on macOS (libtls tests didn't run before the recent
implicit enabling via pki).  Other platforms apparently let accept() fail
if the socket is shutdown/closed in teardown_creds(), macOS apparently
doesn't do that.

src/libtls/tests/suites/test_socket.c

@@ -20,6 +20,7 @@
 #include <unistd.h>
 #include <errno.h>
 
+#include <threading/thread.h>
 #include <processing/jobs/callback_job.h>
 #include <credentials/sets/mem_cred.h>
 
@@ -509,6 +510,7 @@ static job_requeue_t serve_echo(echo_server_config_t *config)
 	identification_t *server, *client = NULL;
 	ssize_t len, total, done;
 	char buf[128];
+	bool oldstate;
 
 	server = identification_create_from_string(config->addr);
 	if (config->cauth)
@@ -516,9 +518,12 @@ static job_requeue_t serve_echo(echo_server_config_t *config)
 		client = server;
 	}
 	sfd = config->fd;
+	thread_cleanup_push((thread_cleanup_t)server->destroy, server);
 	while (TRUE)
 	{
+		oldstate = thread_cancelability(TRUE);
 		cfd = accept(sfd, NULL, NULL);
+		thread_cancelability(oldstate);
 		if (cfd < 0)
 		{
 			break;
@@ -548,7 +553,7 @@ static job_requeue_t serve_echo(echo_server_config_t *config)
 		tls->destroy(tls);
 		close(cfd);
 	}
-	server->destroy(server);
+	thread_cleanup_pop(TRUE);
 
 	return JOB_REQUEUE_NONE;
 }
@@ -575,7 +580,8 @@ static void start_echo_server(echo_server_config_t *config)
 	lib->processor->set_threads(lib->processor, 8);
 
 	lib->processor->queue_job(lib->processor, (job_t*)
-				callback_job_create((void*)serve_echo, config, NULL, NULL));
+				callback_job_create((void*)serve_echo, config, NULL,
+									(callback_job_cancel_t)return_false));
 }
 
 /**