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_init.vim b/src/testdir/test_gui_init.vim
index a8eefa5..ecc8fc9 100644
--- a/src/testdir/test_gui_init.vim
+++ b/src/testdir/test_gui_init.vim
@@ -3,7 +3,7 @@
 
 source shared.vim
 if !CanRunGui()
-  throw 'Skipped, cannot run GUI'
+  throw 'Skipped: cannot run GUI'
 endif
 
 source setup_gui.vim