patch 8.2.0982: insufficient testing for reading/writing files

Problem:    Insufficient testing for reading/writing files.
Solution:   Add more tests. (Yegappan Lakshmanan, closes #6257)
            Add "ui_delay" to test_override() and use it for the CTRL-O test.
diff --git a/src/testdir/test_writefile.vim b/src/testdir/test_writefile.vim
index a00b8a3..9106f86 100644
--- a/src/testdir/test_writefile.vim
+++ b/src/testdir/test_writefile.vim
@@ -434,4 +434,12 @@
   set backupdir& patchmode&
 endfunc
 
+" Test for writing a file using invalid file encoding
+func Test_write_invalid_encoding()
+  new
+  call setline(1, 'abc')
+  call assert_fails('write ++enc=axbyc Xfile', 'E213:')
+  close!
+endfunc
+
 " vim: shiftwidth=2 sts=2 expandtab