Problem: Not enough test coverage in Insert mode. Solution: Add lots of tests. Add test_override(). (Christian Brabandt, closes #1521)
Problem: Using old function name in comment. More functions should start with test_. Solution: Rename function in comment. (Higashi Higashi) Rename disable_char_avail_for_testing() to test_disable_char_avail(). And alloc_fail() to test_alloc_fail().
Problem: Cannot test CursorMovedI because there is typeahead. Solution: Add disable_char_avail_for_testing().
