patch 9.0.0411: only created files can be cleaned up with one call

Problem:    Only created files can be cleaned up with one call.
Solution:   Add flags to mkdir() to delete with a deferred function.
            Expand the writefile() name to a full path to handle changing
            directory.

src/testdir/test_writefile.vim

@@ -950,6 +950,19 @@ func Test_write_with_deferred_delete()
   call assert_equal('', glob('XdefdeferDelete'))
 endfunc
 
+func DoWriteFile()
+  call writefile(['text'], 'Xthefile', 'D')
+  cd ..
+endfunc
+
+func Test_write_defer_delete_chdir()
+  let dir = getcwd()
+  call DoWriteFile()
+  call assert_notequal(dir, getcwd())
+  call chdir(dir)
+  call assert_equal('', glob('Xthefile'))
+endfunc
+
 " Check that buffer is written before triggering QuitPre
 func Test_wq_quitpre_autocommand()
   edit Xsomefile