Fix #1215: Add periodic writes for task-set result files

[saurabh12nxf]

Summary

This PR implements periodic writes for task-set-specific result files when using --results-per-taskset, addressing issue #1215.

Previously, task-set files were only written at the end of benchmark execution, which meant:

• No intermediate results were available during long-running benchmarks
• All results were lost if BenchExec crashed before completion

This change makes task-set files behave consistently with rundefinition files by writing them periodically (every 60 seconds) during execution.

Changes Made

• benchexec/outputhandler.py:
  • Added tracking for task-set (block) XML files in output_before_run_set() method
  • Implemented periodic write logic in output_after_run() method
  • Files are written incrementally using _write_rough_result_xml_to_file() every 60 seconds
  • Final files are written with proper formatting using _write_pretty_result_xml_to_file() at the end

Benefits

1. Crash resilience: Results are preserved even if BenchExec crashes
2. Progress monitoring: Users can view partial results during execution
3. Consistent behavior: Task-set files now behave like rundefinition files
4. Minimal overhead: Writes occur at most once per 60 seconds per task-set
5. Backward compatible: No changes to existing behavior when both file types are enabled

Testing

• ✅ Syntax and import checks passed
• ✅ Existing unit tests pass (no regressions)
• ✅ Functional testing with example benchmarks confirms task-set files are created and updated periodically
• ✅ XML structure verified to be correct

Implementation Details

The implementation follows existing patterns in the codebase:

• Uses the same 60-second interval as rundefinition files
• Maintains thread safety with existing print_lock mechanism
• Handles edge cases (duplicate block names, empty blocks)
• Reuses existing methods (runs_to_xml, _write_rough_result_xml_to_file)

Closes #1215