Add test workflow for legacy opmath

[lillian542]

Currently, the tests only run with the new opmath, so won't know if we introduced a bug that breaks the legacy opmath behaviour while it is still in its deprecation cycle.

This PR creates a kwarg, disable-opmath, that can be passed when running the tests, and a corresponding workflow that runs tests with that kwarg set to True. This can also be used locally, i.e. python -m pytest tests/ --disable-opmath=True

Right now, it runs with CI on the PR, but once the tests are all passing, the line triggering that will be removed and it will only run every 3-4 days, in the middle of the night. Then we will add it to the test matrix (separate PR to modify the plugin test matrix repo).

The changes to .yml files and to the conftest files are all about allowing us to run these additional tests. A few modifications to tests were made to allow them to pass with both legacy opmath and new opmath.

There is one test currently marked as xfail for new opmath that I would call a bug - I opened an issue here: #5512

[github-actions]

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[lillian542]

[sc-59553]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.66%. Comparing base (9a03cce) to head (89e7d15).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5435      +/-   ##
==========================================
- Coverage   99.67%   99.66%   -0.01%     
==========================================
  Files         406      406              
  Lines       37881    37590     -291     
==========================================
- Hits        37757    37465     -292     
- Misses        124      125       +1     

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[Qottmann]

This looks really good, thanks @lillian542 !

My main concern is the change to qml.ops.Hamiltonian. One of the main reasons we went through the trouble of creating LinearCombination and dispatching qml.Hamiltonian to either class was to be able to leave ops.Hamiltonian untouched to continue to guarantee the performance in the lightning paper.
If there is a way to not have this change I would suggest to go that route.

[trbromley]

CC @maliasadi for the discussion on performance numbers.

[PietropaoloFrisoni]

Thanks again @lillian542!

[Qottmann]

Really good job @lillian542 thanks a lot!

[trbromley]

Thanks @lillian542!

Then we will add it to the test matrix (separate PR to modify the plugin test matrix repo).

Do we have a separate story to track this?