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/test_gui.vim b/src/testdir/test_gui.vim
index d030a6a..7f94e6a 100644
--- a/src/testdir/test_gui.vim
+++ b/src/testdir/test_gui.vim
@@ -2,7 +2,7 @@
 
 source shared.vim
 if !CanRunGui()
-  throw 'Skipped, cannot run GUI'
+  throw 'Skipped: cannot run GUI'
 endif
 
 source setup_gui.vim