Transient test failure in TestQuantumShannonDecomposer

[jakelishman]

Environment

• Qiskit Terra version: main (post-0.21rc1) @ f3877a2
• Python version: 3.9
• Operating system: Linux

What is happening?

There was a test failure in the nightly cron job which looks like it was for the quantum Shannon decomposer added in #7907. This seems transient / rarely flaky, since most tests in the same job didn't fail, and I don't remember having seen this same test failure in PR CI before now. See comment with full logs: #7864 (comment).

How can we reproduce the issue?

It's likely transient, but the traceback from the logs indicates that the follow test may produce the error:

test.python.quantum_info.test_synthesis.TestQuantumShannonDecomposer.test_SO3_decomposition_l2_a1_opt
-----------------------------------------------------------------------------------------------------

Captured traceback:
~~~~~~~~~~~~~~~~~~~
    Traceback (most recent call last):

      File "/tmp/terra-tests/test/python/quantum_info/test_synthesis.py", line 1423, in test_SO3_decomposition_l2_a1_opt
    self.assertTrue(np.allclose(mat, Operator(ccirc).data))

      File "/opt/hostedtoolcache/Python/3.9.13/x64/lib/python3.9/unittest/case.py", line 688, in assertTrue
    raise self.failureException(msg)

    AssertionError: False is not true

What should happen?

We need to know the cause first - it could be that the tolerance of the test is too high for acceptable decompositions, or it could have been a failure in the decomposer. Either way, we need to squash any transient failure.

Any suggestions?

No response