Short on Testing Libraries

1- Is testing needful? do every developer should learn to include testing and test cases in a project.
2- Market/Industry demand.

3- how to approach, where to start like from beginning or in between of development, which one to focus more Unit, integrated, e2e , or some other.

If it is necessary for developers to know test libraries than it would be really helpful if in future you can make a industry level small project with implementation of testing in frontend and backend.