patch 9.1.1752: tests: need another test for v9.1.1750 behaviour

patch 9.1.1752: tests: need another test for v9.1.1750 behaviour

Problem:  tests: No test when deleting text after autocompletion with preinsert
          did complete an entry
Solution: Verify, that after deletion autocompletion does not reinsert
          the deleted text. Note: the actual issue was fixed with v9.1.1750.
          (Girish Palya)

Pre v9.1.1750 behaviour:
When autocomplete is enabled with *preinsert*, deleting text after selecting a
longer match could cause unintended reinsertion, e.g.:

- Matches available: "foo" and "foobar".
- User selects "foobar" with Ctrl-N
- User deletes characters back to "foo".
- Autocomplete then incorrectly re-inserts "bar", preventing deletion past "foo".

v9.1.1750 removes the unwanted reinsertion so text can be deleted correctly.

closes: #18259

Signed-off-by: Girish Palya <girishji@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>