Mark back a qubitunitary gradient test as xfailed

[paul0403]

Context: In PR #778, the support for complex matrices under qubitunitary was added.

It was mistakenly belived that an old test, test_adj_qubitunitary in test_gradient.py, was marked xfailed because complex under qubitunitary was not supported, and the PR removed the xfailed.

However, the xfailed was because qubitunitary should not be differentiable at all.

Description of the Change: The test is now marked back to xfailed, and a new test for complex numbers under qubitunitary, with no gradients in the test, is added

Benefits:

Possible Drawbacks:

Related GitHub Issues:

[sc-66151]

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Hello. You may have forgotten to update the changelog! Please edit doc/changelog.md on your branch with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.