Arm unittest refactor of Add and Conv2D test cases

[AdrianLundell]

This is a showcase of multiple improvements which can be done across all tests:

1. Move pipeline definition from tests to a general (flexible) pipeline class

• Define a default step of stages using e.g. TosaPipelineBI()
• Add custom config or debug stages using helper functions s.a. .change_args(), .add_stage(), .dump_artifact() etc.
• Run the full pipeline using .run()

2. Move towards a pure pytest approach to remove dependencies on unittest and parametrize
3. Separate tests running on FVP from tests not running on FVP rather than configuring this from the command line

• FVP tests are skipped if not installed
• To filter out tests one may instead use pytest markers/name filtering
• This should give a clearer picture of what has been tested

4. Introduces one favored way of marking tests as xfails, in the parameterize decorator

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/7541

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❌ 1 New Failure

As of commit 1795dff with merge base a29dc49 ():

NEW FAILURE - The following job has failed:

• pull / unittest / macos / macos-job (gh)
  backends/xnnpack/test/ops/test_add.py::TestAdd::test_qs8_add3

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[AdrianLundell]

@digantdesai Curious to see what you think about this, in particular this removes unittest support in favour of a simpler pytest-only setup + it introduces one extra layer of complexity by wrapping test stages in the new test_pipeline.