patch 8.1.1544: some balloon tests don't run when they can

Problem:    Some balloon tests don't run when they can.
Solution:   Split GUI balloon tests off into a separate file. (Ozaki Kiichi,
            closes #4538)  Change the feature check into a command for
            consistency.
diff --git a/src/testdir/Make_all.mak b/src/testdir/Make_all.mak
index 9e551c1..eb014a9 100644
--- a/src/testdir/Make_all.mak
+++ b/src/testdir/Make_all.mak
@@ -65,6 +65,7 @@
 	test_backspace_opt \
 	test_backup \
 	test_balloon \
+	test_balloon_gui \
 	test_behave \
 	test_blob \
 	test_blockedit \
@@ -297,6 +298,7 @@
 	test_autoload.res \
 	test_backspace_opt.res \
 	test_balloon.res \
+	test_balloon_gui.res \
 	test_blob.res \
 	test_blockedit.res \
 	test_breakindent.res \