Fix flakiness in "accepts own signatures" test

[AndrewFerr]

Every now and then, the test "OlmMachine › verifyBackup › accepts own signatures" in tests/machine.test.js fails in CI, but only for Node 20. Here's an example.

AFAICT the test awaits everything that it needs to, so something else must be going wrong here. This should be fixed not only to have reliable CI, but to see if this indicates is a real user-impacting issue.

[AndrewFerr]

Looks like it happens in Node 19 too: https://github.com/matrix-org/matrix-rust-sdk-crypto-nodejs/actions/runs/5731568175/job/15532744078#step:8:49

[AndrewFerr]

And in Node 18: https://github.com/matrix-org/matrix-rust-sdk-crypto-nodejs/actions/runs/5739890538/job/15556516262?pr=14#step:8:49

I'm beginning to suspect that this test's flakiness has nothing to do with the Node version.