Fix random test failure in TestDigestComputerListener.testDigest

In extremely rare cases (only seen once) we have this failure:

org.junit.ComparisonFailure: expected:<[CJz5xUykO51gRRCIQadZ9dL20NPDd/O0yVBEgP13Skg=]> but was:<[53f53fc3022a815ced06634e9a57a53b]>
    at org.nuxeo.ecm.platform.filemanager.core.listener.tests.TestDigestComputerListener.testDigest(TestDigestComputerListener.java:74)

This happens at exactly 15:35:00 so one hypothesis is that some scheduled event triggered the parallel init of DigestComputer, but its initIfNeeded is not synchronized or thread-safe.

This is all the more likely since this listener isn't properly registered with a list of events it's interested in, so it's called for all events.