Mark and skip particularly slow unit tests

[braised-babbage]

Description

A few tests dominate the runtime of the test suite, and this makes me less likely to run tests. On my system,

== slowest 10 test durations ==
45.54s call     pyquil/tests/test_quantum_computer.py::test_run_symmetrized_readout_error
40.33s call     pyquil/tests/test_reference_density_simulator.py::test_for_negative_probabilities
15.67s call     pyquil/tests/test_operator_estimation.py::test_measure_observables_many_progs
6.24s call     pyquil/tests/test_quantum_computer.py::test_run_pyqvm_noisy
4.40s call     pyquil/tests/test_operator_estimation.py::test_measure_observables_symmetrize_calibrate
4.34s call     pyquil/tests/test_operator_estimation.py::test_2q_unitary_channel_fidelity_readout_error
2.97s call     pyquil/tests/test_operator_estimation.py::test_measure_observables
2.83s call     pyquil/tests/test_operator_estimation.py::test_measure_observables_symmetrize
2.33s call     pyquil/tests/test_operator_estimation.py::test_measure_observables_result_zero_symmetrization_calibration
2.26s call     pyquil/tests/test_operator_estimation.py::test_sic_process_tomo
== 596 passed, 6 skipped, 1 xfailed, 36 warnings in 160.55s (0:02:40) ==

This PR adds a --include-slow-tests flag, as well as a marker pytest.marks.slow for indicating that certain tests are slow. I've flagged the top 3 in the above list. If we decide to move forward with this idea, probably we need to configure gitlab to add the --include-slow-tests (since these things should be tested for a PR, for example).

Checklist

• The above description motivates these changes.
• There is a unit test that covers these changes.
• All new and existing tests pass locally and on Semaphore.
• Parameters have type hints with PEP 484 syntax.
• Functions and classes have useful sphinx-style docstrings.
• (New Feature) The docs have been updated accordingly.
• (Bugfix) The associated issue is referenced above using
  auto-close keywords.
• The changelog is updated,
  including author and PR number (@username, gh-xxx).

[notmgsk]

hm

[karalekas]

wow I drop it from 40 to 3 and it's still too slow for you? :P

[karalekas]

I'd be fine taking this route in the near-term to lower the barrier of entry for developers. You should update the gitlab YAML to run the slow tests, and add some information to the contribution guide about this option and which tests specifically are marked as slow.

[karalekas]

Also need to update changelog ofc

[braised-babbage]

Alternatively we can leave this to users. I was unaware of pytest -k, which allows for an easy command-line deselection of tests, e.g. pytest -k 'not symmetrized_readout and not negative_probabilities'

[karalekas]

if we do this, let's just use the canonical --runslow

[karalekas]

tiny tiny stuff -- also can you move your changelog entry to the bottom of the "improvements and changes" list? each section is pretty much chronological by merge date. otherwise LGTM!

[braised-babbage]

tiny tiny stuff -- also can you move your changelog entry to the bottom of the "improvements and changes" list? each section is pretty much chronological by merge date. otherwise LGTM!

Good catches!

[karalekas]

🎉