patch 8.0.0361: GUI initialisation is not sufficiently tested

Problem:    GUI initialisation is not sufficiently tested.
Solution:   Add the gui_init test. (Kazuki Kuriyama)
diff --git a/src/Makefile b/src/Makefile
index bcfb8cd..227e1df 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -2133,6 +2133,7 @@
 	test_gn \
 	test_goto \
 	test_gui \
+	test_gui_init \
 	test_hardcopy \
 	test_help \
 	test_help_tagjump \