stty: Add 101 unit tests to improve coverage to 43.76%

Add comprehensive unit test coverage for stty.rs core functions:

- Flag struct methods and builder pattern (7 tests)
- Control character parsing and formatting (19 tests)
- All combination settings expansion (26 tests)
- Termios modification functions (13 tests)
- String-to-flag/combo/baud parsing (19 tests)
- TermiosFlag trait implementations (5 tests)
- Helper and utility functions (10 tests)
- Trait implementations (2 tests)

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

Addresses #9061

Author: naoNao89