remove failing test; simplify layerwise ZNE tests

[natestemen]

Description

1. Remove a test that has been randomly failing on CI (https://github.com/unitaryfund/mitiq/commit/e6210e6add29c5aab8b2a8ac54438e93bc14f85c)
2. As part of a Mitiq coding call we found an instance of using an integration test instead of small unit tests. We moved the testing of layerwise folding with all QPROGRAM types into a smaller unit test so we do not need to E2E test the entire workflow with every QPROGRAM instance. (https://github.com/unitaryfund/mitiq/commit/60528db816f61a05ed7bf4bddd66987ebfc4bcff)

Reviewer 👀

I kept a modified version of test_layerwise_folding_with_zne, but struggled with knowing what to assert at the end since we'd like to move away from making assertions about the accuracy of techniques unless there are proofs to back the claims. As far as I understand, the assertions in test_layerwise_folding_with_zne are redundant to the the unit tests in test_layer_scaling.py, but am not 100% sure. WDYT? Should we keep this test? Should it be altered?

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 98.72%. Comparing base (54f7171) to head (c9fb3bc). Report is 43 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2545   +/-   ##
=======================================
  Coverage   98.72%   98.72%           
=======================================
  Files          90       92    +2     
  Lines        4158     4161    +3     
=======================================
+ Hits         4105     4108    +3     
  Misses         53       53           

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.