Partially addresses#5917.

This PR refactors theparse_test_case function inmypy/test/data.py by extracting the logic that parses[out] and[outN] sections into a new private helper,_handle_out_section(). This reduces the size and nesting ofparse_test_case and makes its responsibilities clearer.

Note: This is apartial refactor from#5917. Onlyparse_test_case is updated in this PR; the remaining refactoring work described in the issue is intentionally left for follow-up changes.

No behavior changes are intended. No new tests were added; the existing test suite passes, confirming that this refactor is functionally neutral.