[Capture] Improve unitary_to_rot tests

[andrijapau]

Context:

After some internal discussion, testing strategies have been updated to follow,

class TestXInterpreter:
    ...

def test_X_plxpr_to_plxpr():
    ...

class TestHigherOrderPrimitives:
    ...

class TestExpandPlxprTransform:
    ...

Description of the Change:

Added tests to cover,

• Traced arguments as input (wires or unitaries)

Deleted redundant tests. Improved test suite to only test was they are supposed to be testing.

Benefits: Improved testing.

Possible Drawbacks: None.

[sc-84724]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.59%. Comparing base (7c44146) to head (696027d).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6977   +/-   ##
=======================================
  Coverage   99.59%   99.59%           
=======================================
  Files         483      483           
  Lines       46119    46119           
=======================================
  Hits        45934    45934           
  Misses        185      185           

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[PietropaoloFrisoni]

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