Tests for rest-client-reactive URL override feature

[mocenas]

Summary

Implement test coverage for quarkusio/quarkus#43331

Please select the relevant options.

• Bug fix (non-breaking change which fixes an issue)
• Dependency update
• Refactoring
• Backport
• New scenario (non-breaking change which adds functionality)
• This change requires a documentation update
• This change requires execution against OCP (use run tests phrase in comment)

Checklist:

• Methods and classes used in PR scenarios are meaningful
• Commits are well encapsulated and follow the best practices

[fedinskiy]

1. What if we use non-existent url for override? How good/terrible is the error?
2. Does app writes overridden URL into log?

[mocenas]

1. Trying to access non-existing URL causes exception on the service. It is possible to catch it and pretend it is OK, but that might cover some actual failures going on.
2. No it does not not log rewriting URL.

[fedinskiy]

1. I thought, that maybe we should check that error, and that it really happens, to have some non-happy-path tests. Do not know, whether it is a good idea, but please think about it.
2. And what about tracing? Is there any way for an observer to distinguish these calls?

[fedinskiy]

The existing tests are good, but please consider advanced topics suggested in the comment. If you they do not make much sense in this context — you're free to merge this issue.