Fix flaky test LogCleanerTest.testCommitMarkerRemoval

java.lang.AssertionError: expected:<List(4, 5, 6, 7, 8)> but was:<List(3, 4, 5, 6, 7, 8)>
	at org.junit.Assert.fail(Assert.java:88)
	at org.junit.Assert.failNotEquals(Assert.java:834)
	at org.junit.Assert.assertEquals(Assert.java:118)
	at org.junit.Assert.assertEquals(Assert.java:144)
	at kafka.log.LogCleanerTest.testCommitMarkerRemoval(LogCleanerTest.scala:210)