patch 8.0.1162: shared script for tests cannot be included twice

Problem:    Shared script for tests cannot be included twice.
Solution:   Include it where needed, it will "finish" if loaded again.
diff --git a/src/testdir/test_timers.vim b/src/testdir/test_timers.vim
index 768eead..c8b25c6 100644
--- a/src/testdir/test_timers.vim
+++ b/src/testdir/test_timers.vim
@@ -4,9 +4,7 @@
   finish
 endif
 
-if !exists('*WaitFor')
-  source shared.vim
-endif
+source shared.vim
 
 func MyHandler(timer)
   let g:val += 1