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_bufline.vim b/src/testdir/test_bufline.vim
index 2fe6739..f57dd73 100644
--- a/src/testdir/test_bufline.vim
+++ b/src/testdir/test_bufline.vim
@@ -1,5 +1,9 @@
 " Tests for setbufline() and getbufline()
 
+if !exists('*GetVimCommand')
+  source shared.vim
+endif
+
 func Test_setbufline_getbufline()
   new
   let b = bufnr('%')
@@ -30,7 +34,7 @@
   if cmd == ''
     return
   endif
-  call writefile(['call setline(1, "Hello")', 'w Xtest', 'q!'], 'Xscript')
+  call writefile(['call setline(1, "Hello")', 'silent w Xtest', 'q!'], 'Xscript')
   call system(cmd)
   call assert_equal(['Hello'], readfile('Xtest'))