Add standalone test file for Clifford synthesis functions

[ElePT]

Summary

This PR is a spin-off from #12203 with a few minor improvements to the synthesis functions:

• Moving Clifford synthesis tests to a standalone file in test/python/synthesis instead of test/python/quantum_info
• A proposal to avoid cyclic import errors in synthesis files (this allows to freely import DAGCircuit inside qiskit.synthesis in Add support for high level synthesis plugins to return a DAGCircuit #12203, which would normally raise a cyclic import error)
• Adding copy=False to circuit.append

Details and comments

[qiskit-bot]

One or more of the the following people are requested to review this:

• @Cryoris
• @Qiskit/terra-core
• @ajavadia

[coveralls]

Pull Request Test Coverage Report for Build 8982134163

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 39 of 40 (97.5%) changed or added relevant lines in 9 files are covered.
• 75 unchanged lines in 8 files lost coverage.
• Overall coverage decreased (-0.004%) to 89.629%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit/circuit/library/pauli_evolution.py	3	4	75.0%

Files with Coverage Reduction	New Missed Lines	%
qiskit/circuit/library/generalized_gates/unitary.py	1	91.46%
crates/qasm2/src/expr.rs	1	94.03%
qiskit/synthesis/clifford/clifford_decompose_ag.py	2	96.15%
crates/qasm2/src/lex.rs	3	91.86%
qiskit/converters/init.py	7	72.0%
crates/accelerate/src/sparse_pauli_op.rs	18	93.27%
crates/qasm2/src/parse.rs	18	96.69%
crates/qasm3/src/circuit.rs	25	78.06%

Totals
Change from base Build 8966020047:	-0.004%
Covered Lines:	62258
Relevant Lines:	69462

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💛 - Coveralls

[alexanderivrii]

Thank you very much -- this makes the code cleaner and more efficient (in particular avoiding to copy the subcircuits in clifford synthesis algorithms). My only real suggestion was to move the random clifford generation circuit directly to Qiskit (and remove its support for V and W gates that are not available in the standard library). Of course, that also means that you would need the release notes :).

[alexanderivrii]

Thank you, LGTM!