vim-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 vim/vim#4202)
690a905a01

src/nvim/testdir/test_signals.vim

@@ -4,6 +4,11 @@ if !has('unix')
   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)