Speed up and/or remove redundant unit tests

[Aaron-Robertson]

Pre-Request Checklist

• [x] I checked to make sure that this feature has not already been requested.

Issue Description

Local test times are onerous for Mitiq contributors and for the repository actions. Contribution would be easier if tests run faster.

Based on my own local testing, the 15 slowest tests are	Time	Test
99.69s	mitiq/pec/tests/test_pec.py::test_pec_decorator_qiskit
82.92s	mitiq/zne/tests/test_zne.py::test_with_observable_two_qubits[sample_bitstrings]
49.32s	mitiq/pec/tests/test_pec.py::test_qiskit_noiseless_decomposition_multiqubit[1]
45.16s	mitiq/pec/tests/test_pec.py::test_qiskit_noiseless_decomposition_multiqubit[2]
30.31s	mitiq/zne/tests/test_zne.py::test_with_observable_batched_factory[sample_bitstrings]
28.99s	mitiq/zne/tests/test_zne.py::test_with_observable_adaptive_factory[sample_bitstrings]
26.77s	mitiq/cdr/tests/test_cdr.py::test_mitigated_execute_with_cdr[5-kwargs1-linear_fit_function-pennylane]
26.55s	mitiq/cdr/tests/test_cdr.py::test_mitigated_execute_with_cdr[5-kwargs1-linear_fit_function_no_intercept-pennylane]
26.50s	mitiq/cdr/tests/test_cdr.py::test_mitigated_execute_with_cdr[5-kwargs0-linear_fit_function_no_intercept-pennylane]
25.77s	mitiq/cdr/tests/test_cdr.py::test_mitigated_execute_with_cdr[5-kwargs0-linear_fit_function_no_intercept-qiskit]
25.31s	mitiq/cdr/tests/test_cdr.py::test_mitigated_execute_with_cdr[3-kwargs1-linear_fit_function-pennylane]
24.93s	mitiq/cdr/tests/test_cdr.py::test_mitigated_execute_with_cdr[3-kwargs0-linear_fit_function_no_intercept-pennylane]
24.74s	mitiq/cdr/tests/test_cdr.py::test_mitigated_execute_with_cdr[5-kwargs0-linear_fit_function-qiskit]
24.38s	mitiq/cdr/tests/test_cdr.py::test_mitigated_execute_with_cdr[4-kwargs0-linear_fit_function-pennylane]
24.03s	mitiq/cdr/tests/test_cdr.py::test_mitigated_execute_with_cdr[5-kwargs1-linear_fit_function_no_intercept-qiskit]

Proposed Solution

Look for redundancy in the above tests and remove/reduce wherever possible. Potentially split this issue into two to three sub issues, with the first focused on CDR and the final focused on PyQuil.

Additional References

Add the --durations=N flag to a test run to check local runtimes, where N is the number of ranked test length to track (and 0 returns all test times ranked). I.e. to obtain the above data, the flag is --durations=15 and the full command is pytest -n auto -v --cov=mitiq --cov-report=term --cov-report=xml --ignore=mitiq/interface/mitiq_pyquil --durations=15.

[natestemen]

Some previous work to speed up tests: https://github.com/unitaryfund/mitiq/pull/583

I am working on this, just wanted to add that since it's related.

[Misty-W]

thanks for working on this @natestemen! For purposes of the release I'll close this issue and create a new one for incremental improvements in a future milestone.