tests: add terminal tests for coverage

tests: add terminal tests for coverage

Add tests for src/libcrun/terminal.c:

C unit tests (tests_libcrun_terminal.c) using isolated pty:
- test_cleanup_terminalp_null: NULL cleanup should not crash
- test_terminal_setup_size_invalid_fd: Invalid fd error handling
- test_terminal_setup_size_pty: Set size on isolated pty
- test_set_raw_invalid_fd: Invalid fd error handling
- test_set_raw_pty: Set raw mode on isolated pty
- test_set_raw_no_status: Set raw without saving status
- test_new_terminal: Verify pty creation

Add test_terminal.py to test terminal allocation code in
src/libcrun/terminal.c.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>