ci: replace cargo test with cargo nextest

[nanderstabel]

Description of change

As title.

Nextest is a widely used testing tool for Rust that offers better performance and extra features over the standard cargo test tool.

One feature is that it can detect flaky tests. We do currently have two flaky tests:

• test_qr_code_scanned_handle_siopv2_authorization_request
• verifier_successfully_verifies_eddsa_signed_data

Example of a flaky test messing up our testing: https://github.com/impierce/identity-wallet/actions/runs/13316403841/job/37191398477

Example of a (local) test run with nextest containing a flaky test:

As can be seen, the test is marked as flaky, but all tests still pass.

Links to any relevant issues

Failing job: https://github.com/impierce/identity-wallet/actions/runs/13316403841/job/37191398477

How the change has been tested

Describe the tests that you ran to verify your changes.
Make sure to provide instructions for the maintainer as well as any relevant configurations.

Definition of Done checklist

Add an x to the boxes that are relevant to your changes.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes