This one surfaced as a flake test but turns out to be a legit bug
in FIFOCompaction code. FifoCompaction does not check if an empty
store file is already being compacted by an in-flight compaction
request and still enqueues. It can potentially race with a running
compaction (as in this test case, see jira for the exact exception).
Fixes the edge case and cleans up the test code a bit.
Signed-off-by: Andrew Purtell <apurtell@apache.org>
e59f39e09e