Fix Flaky Test 00731_long_merge_tree_select_opened_files.sh Due to Potential Race Condition

Fix potentially flacky test 00731_long_merge_tree_select_opened_files.sh. It does not fail frequently but we have discovered potential race condition in this test while experimenting with ThreadFuzzer: #9814 See link for the example. #11270 (alexey-milovidov).