Improve the comments and test coverage for compaction progress implementation

When fixing HBASE-26938, we also found the root cause that why sometimes we will get broken compaction progress value, i.e, a negative or more than 100%, it is because multiple compaction can happen at the same time but only use one compaction prorgress, which is not thread safe to track them all.

We have fixed this problem in HBASE-26938, but it is suggested to improve the test coverage of this ares so we will not make a mistake in the future again.