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_diffmode.vim b/src/testdir/test_diffmode.vim
index 8cb5060..9308d04 100644
--- a/src/testdir/test_diffmode.vim
+++ b/src/testdir/test_diffmode.vim
Binary files differ