sync-test logins tests fail with "Crypto Error: Deserialization error"

[mhammond]

Once #5107 lands, running the test fails with "Crypto Error: Deserialization error". A quick look shows that this might be a real bug rather than a test issue. We should look into it deeply as it might be demonstrating a real problem.

To narrow it down, go to the end of https://github.com/mozilla/application-services/blob/main/testing/sync-test/src/logins.rs and comment out the first test, so only test_login_deletes is enabled, then from the sync-test directory execute cargo run -- -s stage logins - that will run just that one test_login_deletes test which fails. It should be impossible for the key to be mismatched because only 1 key is generated by that test.

CC @lougeniaC64

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