patch 8.1.1101: signals test may fail in the GUI

Problem:    Signals test may fail in the GUI.
Solution:   Skip the test for the GUI. (Yee Checng Chin, closes #4202)
diff --git a/src/testdir/test_signals.vim b/src/testdir/test_signals.vim
index 3fea76b..af7b882 100644
--- a/src/testdir/test_signals.vim
+++ b/src/testdir/test_signals.vim
@@ -4,6 +4,11 @@
   finish
 endif
 
+if has('gui_running')
+  " Signals only work for terminals, and won't work for GUI.
+  finish
+endif
+
 source shared.vim
 
 " Test signal WINCH (window resize signal)