ContactsManagerIntegrationTest is flaky

Sometimes, these checks fail on the CI:

• ContactsManagerIntegrationTest > get contacts is initially empty() FAILED
• ContactsManagerIntegrationTest > owner can add contacts() FAILED

Usually, a restart of the pipeline has fixed this. Examples:

• https://code.briarproject.org/briar/briar-mailbox/-/jobs/25250
• https://code.briarproject.org/briar/briar-mailbox/-/jobs/24937
• https://code.briarproject.org/briar/briar-mailbox/-/jobs/25241

I have not been able to reproduce this locally yet.