test: add a test as an example of using did-comm mediation

[mirceanis]

What is being changed

This adds a test that shows how to use the did-comm mediation features.

Quality

Check all that apply:

• [X] I want these changes to be integrated
• [X] I successfully ran pnpm i, pnpm build, pnpm test, pnpm test:browser locally.
• [X] I allow my PR to be updated by the reviewers (to speed up the review process).
• [ ] I added unit tests.
• [X] I added integration tests.

Details

The mediation flow is quite cumbersome and strict. Proposals welcome for improving the state of things.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 65.62500% with 11 lines in your changes are missing coverage. Please review.

Project coverage is 89.95%. Comparing base (c384fb9) to head (aec9fcd). Report is 1 commits behind head on next.

Files	Patch %	Lines
...d-comm/src/protocols/trust-ping-message-handler.ts	52.63%	9 Missing :warning:
packages/remote-server/src/messaging-router.ts	50.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## next #1355 +/- ## ========================================== + Coverage 89.82% 89.95% +0.12% ========================================== Files 176 176 Lines 27678 27695 +17 Branches 2180 2200 +20 ========================================== + Hits 24861 24912 +51 + Misses 2817 2783 -34 ```

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