Fix some tests hung by CompactSplit.requestCompactionInternal ignoring compactionsEnabled check

When compaction is disabled, CompactSplit.compactionsEnabled is set to false and CompactSplit.longCompactions and CompactSplit.shortCompactions is shutdown.
But for CompactSplit.requestCompactionInternal , it only check CompactSplit.compactionsEnabled in CompactSplit.selectCompaction, when selectNow is false, CompactSplit.selectCompaction is not invoked and would submit the compaction task to CompactSplit.shortCompactions,after HBASE-27332, because CompactSplit.shortCompactions is shut down, RejectedExecutionException would throw.

For test such as TestRegionSplit.testSplitTableRegion, because compaction is disabled, this problem would make the new region after split could not open, so it make the test hang.