Small refactoring of unitary synthesis tests

[ElePT]

Summary

This is a small refactoring PR that proposes splitting the UnitarySynthesis test class into two separate classes, one for tests that follow the basis_gates path, which is still in Python, and one for the target path, which is now in Rust. With the current interleaving of target and basis gates tests it's easy to oversee what is covered where, and I believe that as a result we have an improvable coverage of both the Python and Rust UnitarySynthesis code. This PR doesn't fix that but sets the stage for reviewing and adding new tests in an organized way as we work on PRs like #13568.

Details and comments

As I was moving the tests I took the time to squash a few of them into one, as they often just varied in one parameter that could be handled by ddt.

[qiskit-bot]

One or more of the following people are relevant to this code:

• @Qiskit/terra-core

[ElePT]

I removed this test because we originally had it as the backend V1 version of test_coupling_unequal_duration_with_backendv2, it was then migrated to backend v2 and became essentially a duplicate of the other one.

[coveralls]

Pull Request Test Coverage Report for Build 12687926263

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.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 370 unchanged lines in 13 files lost coverage.
• Overall coverage decreased (-0.02%) to 88.942%

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Files with Coverage Reduction	New Missed Lines	%
qiskit/transpiler/passes/optimization/collect_multiqubit_blocks.py	2	98.28%
qiskit/circuit/library/standard_gates/rz.py	3	95.71%
qiskit/circuit/library/standard_gates/rx.py	3	95.95%
qiskit/circuit/library/standard_gates/ry.py	3	95.83%
crates/qasm2/src/lex.rs	4	92.23%
qiskit/primitives/containers/observables_array.py	5	95.0%
crates/accelerate/src/commutation_checker.rs	16	97.41%
qiskit/quantum_info/operators/symplectic/pauli_list.py	16	82.93%
qiskit/quantum_info/operators/symplectic/sparse_pauli_op.py	20	94.58%
crates/accelerate/src/unitary_synthesis.rs	23	93.18%

Totals
Change from base Build 12395850432:	-0.02%
Covered Lines:	79444
Relevant Lines:	89321

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💛 - Coveralls

[ShellyGarion]

LGTM. Thanks @ElePT for the tests refactoring, it's very helpful!

It's not part of this PR, but I would just like to point out that it seems that the XXDecomposer lines inside the unitary_synthesis transpiler pass are not being covered by any of the tests, see the coverage report:
https://coveralls.io/builds/71414685/source?filename=qiskit%2Ftranspiler%2Fpasses%2Fsynthesis%2Funitary_synthesis.py