better detect test succeeded

tests/tftpboot/configuration.nix

@@ -17,7 +17,9 @@ in {
     ../../modules/outputs/ext4fs.nix
     ../../modules/outputs/tftpboot.nix
   ];
+
   config = {
+    hostname = "tftpboot-test";
     # use extracted dts if it was null in the device
     # definition, use actual dts if provided
     hardware.dts.src = lib.mkOverride 500 dts;

tests/tftpboot/script.expect

@@ -20,11 +20,11 @@ expect {
 }
 
 expect {
-   "#" { send "test -f /proc/device-tree/reserved-memory/phram-rootfs*/name && tty \r" }
+   "#" { send "test -f /proc/device-tree/reserved-memory/phram-rootfs*/name && hostname \r" }
    timeout { exit 1 }
 }
 
 expect {
-   "/dev/ttyS0" { exit 0 }
+   "tftpboot-test" { exit 0 }
    timeout { exit 1 }
 }