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_signals.vim b/src/testdir/test_signals.vim
index 365958a..44732df 100644
--- a/src/testdir/test_signals.vim
+++ b/src/testdir/test_signals.vim
@@ -1,7 +1,7 @@
 " Test signal handling.
 
 if !has('unix')
-  throw 'Skipped, not on Unix'
+  throw 'Skipped: not on Unix'
 endif
 
 source shared.vim