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/Filelist b/Filelist
index 99e1004..63088a6 100644
--- a/Filelist
+++ b/Filelist
@@ -120,6 +120,7 @@
 		src/testdir/sautest/autoload/*.vim \
 		src/testdir/runtest.vim \
 		src/testdir/summarize.vim \
+		src/testdir/check.vim \
 		src/testdir/shared.vim \
 		src/testdir/screendump.vim \
 		src/testdir/view_util.vim \