test: update and migrate testing-library v5 (#455)

# Motivation

Testing library
[v5.0.0](https://github.com/testing-library/svelte-testing-library/releases/tag/v5.0.0)
introduces a breaking change that changes the returned `container`
position (see issue
[#313](testing-library/svelte-testing-library#313)).

# Changes

- use `baseElement` instead of `container` when our test actually expect
the body to perform search
- adapt `rerender` which are now promises and do not expect the `props`
parameter

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Co-authored-by: github-actions <41898282+github-actions[bot]@users.noreply.github.com>

Author: peterpeterparker