test: 💍 convert app test to spectator and add enhance linting

[NachoVazquez]

PR Checklist

Please check if your PR fulfills the following requirements:

• [ x ] The commit message follows our guidelines: CONTRIBUTING.md#commit
• [ x ] Tests for the changes have been added (for bug fixes / features)
• Docs have 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)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ x ] Other... Please describe:
Add test and linting

What is the current behavior?

Tests on example app were running using TestBed

Issue Number: N/A

What is the new behavior?

Tests now use spectator and more strict linting rules were added.

Does this PR introduce a breaking change?

[ ] Yes
[ x ] No

Other information