Add sort_bench_merge benchmark for sort -m (merge) performance

[nonontb]

Generate benchmark for sort -m :

Generate 500K line from generate_ascii_data and split them into 8 chunks, sort and merge

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/date/date-locale-hour (fails in this run but passes in the 'main' branch)
Note: The gnu test tests/cut/cut-huge-range is now being skipped but was previously passing.
Congrats! The gnu test tests/unexpand/bounded-memory is now passing!
Skip an intermittent issue tests/pr/bounded-memory (was skipped on 'main', now failing)