Investigate flaky unit tests

Before !1697 (merged) was merged, we saw a couple of pipelines failing in the same place:

https://code.briarproject.org/briar/briar/-/jobs/21288

https://code.briarproject.org/briar/briar/-/jobs/21301

The same tests failed after merging to master:

https://code.briarproject.org/briar/briar/-/jobs/21335

Somehow two unit tests seem to have become flaky:

https://code.briarproject.org/briar/briar/-/blob/bcc7a4b93b28b0e252082bdcf4eb6175fd2d66d3/bramble-core/src/test/java/org/briarproject/bramble/db/DatabaseComponentImplTest.java#L297

https://code.briarproject.org/briar/briar/-/blob/bcc7a4b93b28b0e252082bdcf4eb6175fd2d66d3/bramble-core/src/test/java/org/briarproject/bramble/db/DatabaseComponentImplTest.java#L1900