github: replace call for sleep with wait-for-it.sh

[mkmkme]

Previously integration tests invoked sleep 10 to "give time to settle" the environment. In a fast container, waiting for so long is not needed and in a slow one it might not be enough.

This commit changes this line to use ./util/wait-for-it.sh instead [1] to check robustly that the port is being listened to.

Fixes #49.

[1] https://github.com/vishnubob/wait-for-it

Pull Request check-list

Please make sure to review and check all of these items:

• [ ] Do tests and lints pass with this change?
• [ ] Do the CI tests pass with this change (enable it first in your forked repo and wait for the github action build to finish)?
• [ ] Is the new or changed code fully tested?
• [ ] Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
• [ ] Is there an example added to the examples folder (if applicable)?

NOTE: these things are not required to open a PR and can be done afterwards / while the PR is open.

Description of change

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[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 75.12%. Comparing base (b10b338) to head (798569b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #81 +/- ## ======================================= Coverage 75.11% 75.12% ======================================= Files 132 132 Lines 34397 34400 +3 ======================================= + Hits 25837 25842 +5 + Misses 8560 8558 -2 ```

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