start linting test files

[timmysilv]

Context:
pylint doesn't run for our test files for historical-only reasons, which isn't great. A full solution to this build-up of linting errors will take time, but a first step can be taken! We started talking about this today because we found two tests with the same name in some files, and that can cause unexpected behaviour. Idek if pylint catches that, but either way it sparked a conversation.

Description of the Change:

• Adds an extended .pylintrc file for tests specifically, and a whitelist of test files that are currently passing pylint
• Adds a step to the pylint GitHub action to run pylint on those whitelisted test files to ensure they remain linted
• Adds a step to the pylint GitHub action to check if the PR adds any new test files, and if so, ensure that they are whitelisted. This will ensure that all new test files must meet our linting standard before being added to the codebase
• Adds a make lint and make lint-test target to run pylint on the respective parts of the codebase
• Add # pylint:disable comments (and one actual linting fix!) to files in pennylane/ to ensure the above-mentioned make lint passes locally at the time of this PR
  • I'm not sure why I was seeing errors on my machine (using this repo's pylintrc) if CI passes... whatever
  • make lint-test is ugly today - should the target only run on whitelisted files?

Benefits:
Our test files will be much cleaner, more readable, and sometimes more correct!

Possible Drawbacks:

• This is only step 1. There is more work to be done to clean up our tests
• Users may be confused with these new expectations (tests passing, how to whitelist new files)

Other Remarks

• I'm fine to rename/move any of these files, just tried to give them intuitive-sounding and non-intrusive metadata
• I can't figure out how to add the test whitelist to the pre-commit hook - it only accepts a filename regex which I can't get dynamically by reading the file in the pre-commit-config.yml file

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

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[codecov]

Codecov Report

Merging #3600 (9d2f4ff) into master (6c74b0e) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #3600   +/-   ##
=======================================
  Coverage   99.86%   99.86%           
=======================================
  Files         313      313           
  Lines       28075    28075           
=======================================
  Hits        28038    28038           
  Misses         37       37           

Impacted Files	Coverage Δ
pennylane/collections/apply.py	100.00% <ø> (ø)
pennylane/collections/dot.py	100.00% <ø> (ø)
pennylane/data/dataset.py	100.00% <ø> (ø)
pennylane/optimize/adaptive.py	100.00% <ø> (ø)
pennylane/optimize/rotoselect.py	100.00% <ø> (ø)
pennylane/qaoa/cost.py	100.00% <ø> (ø)
pennylane/qaoa/cycle.py	100.00% <ø> (ø)
pennylane/qaoa/mixers.py	100.00% <ø> (ø)
pennylane/math/is_independent.py	100.00% <100.00%> (ø)
...nnylane/templates/layers/particle_conserving_u2.py	100.00% <100.00%> (ø)

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[trbromley]

For new tests, will they have to be added to tests_passing_pylint.txt?

[timmysilv]

@trbromley still thinking about what to do with new tests but maybe

[apotapov]

Nice!

Would be good to give a way for devs to run this locally. Perhaps a shell script?

[apotapov]

lgtm, but would be good to have someone more knowledgeable to approve

[timmysilv]

Here's an example run that fails when I added a new test file but didn't whitelist it, and the passing run once I whitelisted it.