fix bug about comparing key in ob_compaction_test

[Muqi1029]

Problem:

In MiniOB, a key consists of an internal key followed by a seq component. When checking for overlapping keys in a sorted run, we should sort and compare based on the internal key only. This is typically done by using the internal key as the sorting index and then comparing the last key of the previous SSTable with the first key of the current one.

However, if the full key (including seq) is used as the sorting index, the seq component will incorrectly influence the result of the overlap check, potentially leading to false negatives or positives.

What is changed and how it works?

This PR replaces direct string comparison with the obInternalKeyComparator to sort and compare keys correctly, ensuring that only the internal key is considered during overlap checks.

Other information

[CLAassistant]

All committers have signed the CLA.

[nautaa]

LGTM, thanks for your contribution!

[nautaa]

@Muqi1029 please check the cla at the above.

[Muqi1029]

@Muqi1029 please check the cla at the above.

Yes, I've signed it. Thanks for your remind.