[Testing] Speed up Test Suite, Push Code Coverage above 90%

[jpmoutinho]

Main bottlenecks:

1. not using pytest-xdist (TOP prio which has been investigated by both @seitzdom and @m.dagrada only to result in issues with gitlab runners, this however change when we switch to github)
2. shadows/tomography
3. PSR tests in general
4. PSR tests which compute higher order derivatives
5. tests which train models (ML tools)
6. redundant test cases in many mark.parametrize-tests
7. tests which do exactly the same thing, like in test_quantum_models and test_backends for example (test run,sample, expectation for random circuits)

[jpmoutinho]

@dominikandreasseitz maybe some of the above has already been done, could you check and update? Thanks

[RolandMacDoland]

@dominikandreasseitz @jpmoutinho Is it still relevant ? Some of these tests have move in few other places.