Add tests for Flask App

There are no tests for the Flask [app.py](https://github.com/aafre/resume-builder/blob/main/app.py) at the moment. We need to set up testing using `pytest` and cover the most critical scenarios.

We can start with simple tests and then extend them later.

The `docker.dev.api` file can be used to run the application locally using Docker. The app needs to run on Linux, hence, Docker is the preferred way to run the app locally. Look at the [local development steps](https://github.com/aafre/resume-builder?tab=readme-ov-file#local-development) to run via Docker.

It is also imperative that we add tests for the [resume_generator.py](https://github.com/aafre/resume-builder/blob/main/resume_generator.py) script, as that is where the core functionality lies and Flask is just an HTTP wrapper on top of it.