Makefile: add new test targets

Currently, there is no target in the Makefile that runs the full test suite, or launch different types of tests separately.

This MR:

• defines more specialized test targets (test-unit, test-python, test-flextesa, test-tezt, lint-opam-dune)
• the existing target test now runs everything, including python tests and tezt tests
• update the testing doc, and the CI accordingly

The goal of this MR is to:

1. give the ability to the users to only run the framework(s) they use and have configured on their machine (at least be able to run a fast make test-unit)
2. add a little more structure and symmetry to the Makefile (all test targets follow a convention make test-*)

TODO:

• We could do a similar pass to make linting targets more uniform (make lint-* left for a different MR)
• Not clear why make test depends on lint-opam-dune. More generally, we should provide better separation between test and lint.

I'd like to address these two points in a separate MR.