patch 9.0.0525: manually deleting temp test files

Problem:    Manually deleting temp test files.
Solution:   Add the 'D' flag to writefile().

src/testdir/test_digraph.vim

@@ -494,9 +494,8 @@ endfunc
 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