patch 8.1.1581: shared functions for testing are disorganised

Problem:    Shared functions for testing are disorganised.
Solution:   Group finctions in script files. (Ozaki Kiichi, closes #4573)
diff --git a/src/testdir/test_timers.vim b/src/testdir/test_timers.vim
index e67da13..9066134 100644
--- a/src/testdir/test_timers.vim
+++ b/src/testdir/test_timers.vim
@@ -4,7 +4,7 @@
 CheckFeature timers
 
 source shared.vim
-source screendump.vim
+source term_util.vim
 
 func MyHandler(timer)
   let g:val += 1