patch 9.1.1532: termdebug: not enough ways to configure breakpoints

patch 9.1.1532: termdebug: not enough ways to configure breakpoints

Problem:  termdebug: not enough ways to configure breakpoints
Solution: add the termdebug_config['signs'] config setting, rework the
          termdebug test cases (Dimitry Ishenko)

Allow to configure custom breakpoint signs so one can do something like
this:

```vim
let g:termdebug_config['signs'] = ['>1', '>2', '>3', '>4', '>5', '>6', '>7', '>8', '>9']
let g:termdebug_config['sign'] = '>>'
```

where the first 9 breakpoints will have their own signs and the rest
will be the same (>>).

While at it, rework the test for the termdebug plugin:

- Added test for g:termdebug_config['signs'].
- Added test for g:termdebug_config['sign'].
- Moved test for g:termdebug_config['sign_decimal'] into
  Test_termdebug_basic()

closes: #17694

Signed-off-by: Dimitry Ishenko <dimitry.ishenko@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>