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/testdir/Make_ming.mak b/src/testdir/Make_ming.mak
index ab6a60b..61d77d2 100644
--- a/src/testdir/Make_ming.mak
+++ b/src/testdir/Make_ming.mak
@@ -66,8 +66,8 @@
 
 # TODO: find a way to avoid changing the distributed files.
 fixff:
-	-$(VIMPROG) -u dos.vim $(NO_PLUGIN) "+argdo set ff=dos|upd" +q *.in *.ok
-	-$(VIMPROG) -u dos.vim $(NO_PLUGIN) "+argdo set ff=unix|upd" +q \
+	-$(VIMPROG) -u dos.vim $(NO_INITS) "+argdo set ff=dos|upd" +q *.in *.ok
+	-$(VIMPROG) -u dos.vim $(NO_INITS) "+argdo set ff=unix|upd" +q \
 		dotest.in test60.ok test_listchars.ok \
 		test_getcwd.ok test_wordcount.ok
 
@@ -91,7 +91,7 @@
 
 .in.out:
 	-@if exist $*.ok $(CP) $*.ok test.ok
-	$(VIMPROG) -u dos.vim $(NO_PLUGIN) -s dotest.in $*.in
+	$(VIMPROG) -u dos.vim $(NO_INITS) -s dotest.in $*.in
 	@diff test.out $*.ok
 	-@if exist $*.out $(DEL) $*.out
 	@$(MV) test.out $*.out
@@ -107,7 +107,7 @@
 
 bench_re_freeze.out: bench_re_freeze.vim
 	-$(DEL) benchmark.out
-	$(VIMPROG) -u dos.vim $(NO_PLUGIN) $*.in
+	$(VIMPROG) -u dos.vim $(NO_INITS) $*.in
 	$(CAT) benchmark.out
 
 # New style of tests uses Vim script with assert calls.  These are easier
@@ -118,6 +118,16 @@
 
 .vim.res:
 	@echo "$(VIMPROG)" > vimcmd
-	$(VIMPROG) -u NONE $(NO_PLUGIN) -S runtest.vim $*.vim
+	$(VIMPROG) -u NONE $(NO_INITS) -S runtest.vim $*.vim
+	@$(DEL) vimcmd
+
+test_gui.res: test_gui.vim
+	@echo "$(VIMPROG)" > vimcmd
+	$(VIMPROG) -u NONE $(NO_INITS) -S runtest.vim $<
+	@$(DEL) vimcmd
+
+test_gui_init.res: test_gui_init.vim
+	@echo "$(VIMPROG)" > vimcmd
+	$(VIMPROG) -u NONE -U gui_init.vim $(NO_PLUGINS) -S runtest.vim $<
 	@$(DEL) vimcmd