tests: adding unit testing to App class

[luislard]

Please check if the PR fulfills these requirements

• [ ] The commit message follows our guidelines
• [ ] Tests for the changes have been added (for bug fixes/features)
• [ ] Docs have been added/updated (for bug fixes/features)

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...) Adding unit testing to App Class

What is the current behavior? (You can also link to an open issue here) Missing tests

What is the new behavior (if this is a feature change)? Added some tests

Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?) No

Other information: