docs: added section on how to create integration tests and how they work

Purpose of this PR

Integration tests are extremely important to ensure a correct behavior for the Ion shell. Ion shell however has its own way to do integration tests. For this reason integration test in Ion should be documented.

Location of the documentation of integration tests

I have put this section into the CONTRIBUTING.md file because integration tests are relevant to the contributors.

About CI

The job linux:stable of the CI of Ion fails with compiling error.

See this issue for more details.