Add assert_screen_change

Variant of wait_screen_change but dying on no screen change.

testapi.pm

@@ -57,7 +57,8 @@ our @EXPORT = qw($realname $username $password $serialdev %cmd %vars
   upload_asset upload_image data_url assert_shutdown parse_junit_log
   upload_logs
 
-  wait_idle wait_screen_change wait_still_screen wait_serial record_soft_failure
+  wait_idle wait_screen_change assert_screen_change wait_still_screen wait_serial
+  record_soft_failure
   become_root x11_start_program ensure_installed eject_cd power
 
   save_memory_dump save_storage_drives freeze_vm
@@ -396,6 +397,19 @@ sub wait_screen_change(&@) {
     return 0;
 }
 
+=head2 assert_screen_change
+
+  assert_screen_change { CODEREF [,$timeout] };
+
+Run C<CODEREF> with C<wait_screen_change> but C<die> if screen did not change within timeout.
+
+=cut
+
+sub assert_screen_change(&@) {
+    ::wait_screen_change(@_) or die 'assert_screen_change failed to detect a screen change';
+}
+
+
 =head2 wait_still_screen
 
 =for stopwords stilltime