patch 9.0.0525: manually deleting temp test files

Problem:    Manually deleting temp test files.
Solution:   Add the 'D' flag to writefile().
diff --git a/src/testdir/test_digraph.vim b/src/testdir/test_digraph.vim
index 8420b53..a8a55a7 100644
--- a/src/testdir/test_digraph.vim
+++ b/src/testdir/test_digraph.vim
@@ -494,9 +494,8 @@
 func Test_loadkeymap_error()
   CheckFeature keymap
   call assert_fails('loadkeymap', 'E105:')
-  call writefile(['loadkeymap', 'a'], 'Xkeymap')
+  call writefile(['loadkeymap', 'a'], 'Xkeymap', 'D')
   call assert_fails('source Xkeymap', 'E791:')
-  call delete('Xkeymap')
 endfunc
 
 " Test for the characters displayed on the screen when entering a digraph