Test all operators with assert_valid, or fail with a reason

[timmysilv]

Context:
Continuation of the work I started in #4922 (this is part 1 of the future work listed in that PR)

Description of the Change:

1. Implement _flatten for BlockEncode and SpecialUnitary to pytreeify those types
2. Organize tests/ops/functions/conftest.py to better parametrize tests. The parametrization fixtures are easier to make using the following:

• _INSTANCES_TO_TEST is a list of operator instances that couldn't be auto-generated, but do pass validation
• _INSTANCES_TO_FAIL is a list of tuples ((Operator, Type[Exception])) where the first element is an operator instance that will fail validation, and the second element is the exception type raised during validation. Ideally, it also comes with a comment explaining what's failing/why it should fail.

All test collection logic is now in conftest.py, making the tests much smaller and simpler. The only "extra" logic in the tests is the xfail I added for all qutrit ops before auto-generation. Now there are 3 tests:

1. test_generated_list_of_ops, which auto-generates instances of every Operator type and asserts its validity
2. test_explicit_list_of_ops, which tests the validity of manual operator instances
3. test_explicit_list_of_failing_ops, which ensures that invalid operator instances fail to validate as expected

Benefits:
Every single pennylane operator (except for templates) is (in)validated in a single test file, and most of it is done automatically.

Possible Drawbacks:
Yet another test file that needs maintenance.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f44f928) 99.67% compared to head (ae2916c) 99.67%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4978      +/-   ##
==========================================
- Coverage   99.67%   99.67%   -0.01%     
==========================================
  Files         392      392              
  Lines       35556    35303     -253     
==========================================
- Hits        35442    35188     -254     
- Misses        114      115       +1     

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[albi3ro]

I obviously like having a list of all the things we need to fix. Though I am distracted by wanting to fix all those issues immediately, the point of isn't to fix the issues, but just to keep a list of the things we need to fix.

[mudit2812]

Looks good, but lets wait on merging this until after the RC-master merge and fixing the consequent IsingXX PickleError (or lack thereof).