Fix race conditions
Because of the nature of Rust tests, the nutshell integration test has a race
condition. Triggering the issue in the local environment was hard, but it
happens more often in #509 because all the keys are read from memory without
blocking the database.
The error happens because, by default, Rust tests run in parallel, and maybe by
design or by mistake, the external wallet did not increase its counter until
the completion of the minting. This led to some tests having the same blinded
messages, and the fastest test would mint successfully. The slowest test would
result in the CDK failing to sign duplicated blinded messages.
In any case, the fix is on the rust side, running the tests sequentially.
