refactor(example-app): migrate unit tests to Vitest

[jase88]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/main/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[x] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

example-app uses jest as test runner.

Closes #5082

What is the new behavior?

example-app usesvitest as test runner.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

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[timdeschryver]

This looks good, thanks @jase88 !

[rainerhahnekamp]

Yup, is alright for me as well. Good work.