patch 8.0.0440: not enough test coverage in Insert mode

Problem:    Not enough test coverage in Insert mode.
Solution:   Add lots of tests.  Add test_override(). (Christian Brabandt,
            closes #1521)
diff --git a/src/testdir/test_cursor_func.vim b/src/testdir/test_cursor_func.vim
index 4b2a9b8..3553541 100644
--- a/src/testdir/test_cursor_func.vim
+++ b/src/testdir/test_cursor_func.vim
@@ -44,9 +44,9 @@
   new
   call setline(1, ['func()', '{', '}', '----'])
   autocmd! CursorMovedI * call s:Highlight_Matching_Pair()
-  call test_disable_char_avail(1)
+  call test_override("char_avail", 1)
   exe "normal! 3Ga\<Down>X\<Esc>"
-  call test_disable_char_avail(0)
+  call test_override("char_avail", 0)
   call assert_equal('-X---', getline(4))
   autocmd! CursorMovedI *
   quit!