stty: Add 101 unit tests and 16 integration tests to improve coverage

Add comprehensive test coverage for stty:

Unit tests (src/uu/stty/src/stty.rs):
- 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)

Integration tests (tests/by-util/test_stty.rs):
- Help and version output validation (2 tests)
- Invalid argument handling (3 tests)
- Control character overflow validation (2 tests)
- Grouped flag removal validation (1 test)
- File argument error handling (1 test)
- Conflicting print modes (1 test)
- Additional TTY-dependent tests (6 tests, ignored in CI)

Unit test coverage improved from 0% to 43.76% (207/473 lines).
Integration tests validate argument parsing and error handling.

Addresses #9061

Author: naoNao89