Improve stability of ProgressLoggerTest

Improve stability of ProgressLoggerTest

in order to avoid :
log_at_fixed_intervals(org.sonar.core.util.ProgressLoggerTest)
java.util.ConcurrentModificationException: null
	at java.util.ArrayList$Itr.checkForComodification(ArrayList.java:901)
	at java.util.ArrayList$Itr.next(ArrayList.java:851)
	at org.sonar.api.internal.google.common.collect.AbstractMapBasedMultimap$WrappedCollection$WrappedIterator.next(AbstractMapBasedMultimap.java:486)
	at java.util.Spliterators$IteratorSpliterator.tryAdvance(Spliterators.java:1812)
	at java.util.stream.ReferencePipeline.forEachWithCancel(ReferencePipeline.java:126)
	at java.util.stream.AbstractPipeline.copyIntoWithCancel(AbstractPipeline.java:498)
	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:485)
	at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
	at java.util.stream.FindOps$FindOp.evaluateSequential(FindOps.java:152)
	at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
	at java.util.stream.ReferencePipeline.findFirst(ReferencePipeline.java:464)
	at org.sonar.core.util.ProgressLoggerTest.hasInfoLog(ProgressLoggerTest.java:71)
	at org.sonar.core.util.ProgressLoggerTest.log_at_fixed_intervals(ProgressLoggerTest.java:44)