test: use reserved testing domain example.test (RFC 2606)

[daniel-mader]

Description of change

Error message

The test test_qr_code_scanned_handle_siopv2_authorization_request runs successfully locally, but fails in CI.

Expected: Error while fetching configuration: failed to parse response into JSON value
==> The external system could be reached, but the response cannot be parsed into a JSON value

Actual: Error while fetching configuration: failed to get response from resource url: https://example.com/.well-known/did-configuration.json
==> The external system could not be reached at all

Root cause

One of the problems for this inconsistency is that an actually existing domain is used for many integration tests: example.com. It thereby needs to be treated as "external dependency" - even if it's just for testing purposes.
Depending on whether the test execution environment has internet access or not, the HTTP responses when trying to fetch from that resource are differing.

Solution

There are TLDs specifically reserved for testing, such as .test or .example.

Links to any relevant issues

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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

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• 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