Fix cuquantum test bug in test_backends_clifford.py

[renatomello]

Checklist:

• Reviewers confirm new code works as expected.
• Tests are passing.
• Coverage does not decrease.
• Documentation is updated.

[alecandido]

@renatomello I'd say that a better fix would be to decouple the Clifford engine from the other backends.
By now, it is not really reusing anything of the backends. So, I'd just parametrize tests on engine, and spell out which are the available engines.

However, if you prefer to have this immediately in master as a hotfix, I have nothing against it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (7190eae) to head (e8585d4).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #1236   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           72        72           
  Lines        10480     10480           
=========================================
  Hits         10480     10480           

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

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