patch 8.0.0456: typo in MinGW test makefile

Problem:    Typo in MinGW test makefile.
Solution:   Change an underscore to a dot. (Michael Soyka)
diff --git a/src/testdir/Make_ming.mak b/src/testdir/Make_ming.mak
index 7e13c0f..dbf94f0 100644
--- a/src/testdir/Make_ming.mak
+++ b/src/testdir/Make_ming.mak
@@ -129,7 +129,7 @@
 
 test_gui_init.res: test_gui_init.vim
 	@echo "$(VIMPROG)" > vimcmd
-	$(VIMPROG) -u gui_preinit_vim -U gui_init.vim $(NO_PLUGINS) -S runtest.vim $<
+	$(VIMPROG) -u gui_preinit.vim -U gui_init.vim $(NO_PLUGINS) -S runtest.vim $<
 	@$(DEL) vimcmd
 
 opt_test.vim: ../option.c gen_opt_test.vim