Fix PauliOp matmul with iI & add PauliOp test

[king-p3nguin]

Summary

• Resolve PauliOp matmul gives incorrect result with iI #8680
• Add tests for PauliOp

Details and comments

• Simply deleting the following part works.
  https://github.com/Qiskit/qiskit-terra/blob/4b860332a69ebfa8a4b52e0ab5dd6c753db5c39e/qiskit/opflow/primitive_ops/pauli_op.py#L157-L159

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[coveralls]

Pull Request Test Coverage Report for Build 3940522074

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.002%) to 84.833%

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Totals
Change from base Build 3940320475:	0.002%
Covered Lines:	65658
Relevant Lines:	77397

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💛 - Coveralls

[ikkoham]

Nice catch. Thanks. The change looks good to me.

Could you move the test to the following file instead of creating new one?
https://github.com/Qiskit/qiskit-terra/blob/main/test/python/opflow/test_op_construction.py

[king-p3nguin]

Thank you for reviewing! I transferred the tests.

[ikkoham]

Thank you for achieving my comment! LGTM!