Add comprehensive tests for -t custom toplevel flag

Add comprehensive tests for -t custom toplevel flag

- Create Prolog integration tests in src/tests/custom_toplevel.pl
- Add CLI test configuration in tests/scryer/cli/src_tests/custom_toplevel_tests.toml
- Tests verify:
  * -t halt terminates after initialization
  * Custom toplevels can be user-defined predicates
  * Toplevel receives control after initialization completes
  * Default behavior is REPL when no -t specified
- All tests pass successfully

Following TESTING_GUIDE.md three-layer testing approach:
- Layer 2: Prolog integration tests with test_framework
- Layer 3: CLI snapshot tests with .toml configuration

Co-Authored-By: J.J.'s Robot <[email protected]>