Add prelim support for test-level markers

[mlxd]

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Context: This PR introduces new test-level markers for PennyLane, which can begin use for indication of where interactions take-place in the given suite. This forms part of the work on improving testing across the packages
[sc-61416]

Description of the Change: Add 3 new test-level markers for pytest: unit_test, integration_test and system_test. Some sample tests that fit into the 3 categories are added for examination purposes.

Benefits: This will help us to structure our test-suite to better reflect localised tests from intermodule and interpackage tests.

Possible Drawbacks: 3 new markers to work with.

Related GitHub Issues:

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.67%. Comparing base (b7b4b75) to head (34a1836).
Report is 262 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5517      +/-   ##
==========================================
- Coverage   99.68%   99.67%   -0.01%     
==========================================
  Files         415      416       +1     
  Lines       38898    38745     -153     
==========================================
- Hits        38774    38620     -154     
- Misses        124      125       +1     

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[dwierichs]

Had two small ideas :)

Otherwise looks good to me!

[PietropaoloFrisoni]

Thanks, @mlxd, this looks great. It is most probably going to be quite useful in the future

[dwierichs]

Thanks @mlxd 🎉 Looking forward to using them soon!