AbstractConcurrentDataBrokerTest causes deadlocks which AbstractDataBrokerTest did not

The AbstractConcurrentDataBrokerTest, introduced in CONTROLLER-1578 to replace the AbstractDataBrokerTest, while working well in a number of tests I've been using it since, seems to cause deadlocks in some rare cases.

For example, https://git.opendaylight.org/gerrit/#/c/54353/ replaces AbstractDataBrokerTest in Daexim by AbstractConcurrentDataBrokerTest, and causes DataExportImportAppProviderTest to deadlock... Attached is a thread dump.

Tali Ben-Meir seems to have hit a similar issue in a not-yet-published new test she's writing for project Federation, according to private email exchange with a thread dump that looks similar.