Tracker: parameterizing pytest

[tias]

Goal: test-suite can run all relevant test on relevant solvers, through parameterizing the tests.

Behaviour:

• python -m pytest ./tests runs all tests with their default solvers (generic tests with default solver, solver-specific test with that solver if its installed)
• python -m pytest ./tests --solver exact runs all tests (that take solver as a argument) with that solver, and its solver-specific tests

Current status:

• test_constraints: fully parameterized
• test_solvers: fully parameterized
• rest: unknown

Tracker:

• Original impossible to merge PR, for reference: solver parameterised pytest #780
• Updates on testing global constraints #870