BookieStorageThresholdTest#testStorageThresholdCompaction is flaky on CI

[sijie]

Problem

2018-07-20\T\08:51:54.182 [ERROR] org.apache.bookkeeper.bookie.BookieStorageThresholdTest.testStorageThresholdCompaction(org.apache.bookkeeper.bookie.BookieStorageThresholdTest)
2018-07-20\T\08:51:54.182 [ERROR]   Run 1: BookieStorageThresholdTest.testStorageThresholdCompaction:229 Found entry log file ([0,1,2].log. They should have been compacted/tmp/ledger2356933004759883304test2/current
2018-07-20\T\08:51:54.183 [ERROR]   Run 2: BookieStorageThresholdTest.testStorageThresholdCompaction:229 Found entry log file ([0,1,2].log. They should have been compacted/tmp/ledger7058711915875226551test2/current
2018-07-20\T\08:51:54.183 [ERROR]   Run 3: BookieStorageThresholdTest.testStorageThresholdCompaction:229 Found entry log file ([0,1,2].log. They should have been compacted/tmp/ledger4884835867022850349test2/current