patch 8.1.2421: test88 is old style

Problem:    Test88 is old style.
Solution:   Turn into a new style test. (Yegappan Lakshmanan, closes #5347)
diff --git a/src/Makefile b/src/Makefile
index 6339a52..1fe94f5 100644
--- a/src/Makefile
+++ b/src/Makefile
@@ -2257,7 +2257,7 @@
 	test52 test59 \
 	test64 test69 \
 	test70 test72 \
-	test86 test87 test88 \
+	test86 test87 \
 	test95 test99:
 	cd testdir; rm -f $@.out; $(MAKE) -f Makefile $@.out VIMPROG=../$(VIMTESTTARGET) $(GUI_TESTARG) SCRIPTSOURCE=../$(SCRIPTSOURCE)