stty: Add essential unit tests and integration tests to improve coverage

- Added 11 essential unit tests for complex internal functions:
  * Control character parsing (string_to_control_char)
  * Control character formatting (control_char_to_string)
  * Combination settings expansion (combo_to_flags)
  * Terminal size parsing with overflow handling (parse_rows_cols)
  * Sane control character defaults (get_sane_control_char)

- Added 16 integration tests for command behavior:
  * Help/version output validation
  * Invalid argument handling
  * Control character overflow validation
  * Grouped flag removal validation
  * File argument error handling
  * Conflicting print modes
  * TTY-dependent tests (marked as ignored for CI)

Unit tests focus on complex parsing logic that's difficult to test via
integration tests. Integration tests validate actual command behavior.

Coverage improved from 0% to 43.76% (207/473 lines).

Fixes #9061

Author: naoNao89