HDDS-7588. Intermittent failure in TestObjectStoreWithLegacyFS#testFlatKeyStructureWithOBS

[kaijchen]

What changes were proposed in this pull request?

Quoted from javadoc of testFlatKeyStructureWithOBS():

Test verifies that OBS bucket keys should create flat key-value
structure and intermediate directories shouldn't be created even
if the OZONE_OM_ENABLE_FILESYSTEM_PATHS flag is TRUE.

It turns out that the previous fix #4040 was not correct, in term of matching the original intention of this test.
I have reverted c119557 on master.

This PR is a 2nd attempt to fix the flaky test TestObjectStoreWithLegacyFS#testFlatKeyStructureWithOBS,
without changing the original intention (so it matches with the javadoc).

What is the link to the Apache JIRA

https://issues.apache.org/jira/browse/HDDS-7588

How was this patch tested?

Full CI: https://github.com/kaijchen/ozone/actions/runs/4221363513
Repeat 1000x: https://github.com/kaijchen/ozone/actions/runs/4221392332

[adoroszlai]

Thanks @kaijchen for the fix.

[kaijchen]

Thanks @adoroszlai for the review.