patch 8.0.1055: bufline test hangs on MS-Windows

Problem:    Bufline test hangs on MS-Windows.
Solution:   Avoid message for writing file.  Source shared.vim when running
            test individually.
diff --git a/src/testdir/test_timers.vim b/src/testdir/test_timers.vim
index 142bfc8..c9171b7 100644
--- a/src/testdir/test_timers.vim
+++ b/src/testdir/test_timers.vim
@@ -4,6 +4,10 @@
   finish
 endif
 
+if !exists('*WaitFor')
+  source shared.vim
+endif
+
 func MyHandler(timer)
   let g:val += 1
 endfunc