Use pytest-rng generated seeds for stochastic tests

[astralcai]

Context:

The PennyLane CI suffers from sporadic failures due to stochasticity of test cases. In order to save CI runtime, we have seeded many tests. However, this approach introduces the risk of a test case passing due to a special seed, potentially hiding bugs that could have been discovered. Therefore, it is beneficial to periodically update the seeds to review such tests.

Description of the Change:

• The test suite uses pytest-rng to handle seed generation. A seed fixture is now available to use for every test case. If seeding is required, please use the seed provided by pytest-rng

def test_some_function(..., seed):
    dev = qml.device("default.qubit", seed=seed)

• A local_salt test marker has been added that allows you to modify the seed locally for a particular test case if the generated seed happens to make your test fail.

@pytest.mark.local_salt(42)
def test_some_function(..., seed):
    dev = qml.device("default.qubit", seed=seed)

• All tests that uses local seeds has been updated to use the new seeding approach.

• An rng_salt has been added to pytest.ini. This controls the seed generation for the entire test suite. We want to periodically change this rng_salt which updates all the seeds across the test suite.

• The fixture that sets the global seed for every test case has been removed, and another fixture is added that restores the global seed after each test in case the test modifies the global seed.

Benefits:

We will be able to catch bugs hidden in tests that are passing only because of a magic seed that was set locally.

Possible Drawbacks:

This is a chore to maintain this seed an ensure that all tests pass with the new salt when it is updated.

Related Shortcut Story:

[sc-74294]

[github-actions]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.39%. Comparing base (661dd52) to head (ff4b102).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #6435   +/-   ##
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  Coverage   99.38%   99.39%           
=======================================
  Files         452      452           
  Lines       42789    42787    -2     
=======================================
- Hits        42527    42526    -1     
+ Misses        262      261    -1     

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[mudit2812]

Looks good to me. We're still maintaining a "global" seeding behaviour in the sense that we're globally setting a value that affects local seeds. I'm a big fan of this approach, and it's making our tests much more thread safe. I'm happy to approve this PR in its current state, no blocking comments.

[JerryChen97]

Thanks a lot for this whole work @astralcai ! It's time for us to get rid of magic power in our previous testings involving any random sampling; I do think we are on the correct track, although probably more perspectives from another senior instead of me should be included.
Also do we have methods to track the traces of salts? Like is it possible to collect the salts for those false negativities.