patch 9.0.0381: writefile test leaves files behind

Problem:    Writefile test leaves files behind.
Solution:   Fix the file names of files to be deleted. (Dominique Pellé,
            closes #11056)
diff --git a/src/testdir/test_writefile.vim b/src/testdir/test_writefile.vim
index 4c1311d..dddc5c9 100644
--- a/src/testdir/test_writefile.vim
+++ b/src/testdir/test_writefile.vim
@@ -75,8 +75,8 @@
   call assert_match('CONVERSION ERROR', output)
   let output = execute('write! ++enc=ucs-2 Xwfcfile')
   call assert_match('CONVERSION ERROR', output)
-  call delete('Xfilz~')
-  call delete('Xfily~')
+  call delete('Xwfcfilz~')
+  call delete('Xwfcfily~')
   %bw!
 
   call delete('Xwfcfile')