XNNPACK pytest cleanup

[kirklandsign]

Summary:

• Test file names should start with test_ to be collected by pytest
• Add ops test to pytest.ini
• Avoid Tester to be collected

Differential Revision: D57463419

[pytorch-bot[bot]]

:link: Helpful Links

:test_tube: See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/3646

• :page_facing_up: Preview Python docs built from this PR

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:x: 1 New Failure, 1 Unrelated Failure

As of commit 1e26ca1881ecdb7d0a17444ff8ec1c80e5417a48 with merge base 5c70121323fd3580438924c361364f5564e64efe ():

NEW FAILURE - The following job has failed:

* [pull / unittest / macos (buck2) / macos-job](https://hud.pytorch.org/pr/pytorch/executorch/3646#25075704160) ([gh](https://github.com/pytorch/executorch/actions/runs/9119723068/job/25075704160)) `backends/xnnpack/test/models/test_llama2_et_example.py::TestLlama2ETExample::test_f32`

FLAKY - The following job failed but was likely due to flakiness present on trunk:

* [pull / unittest / linux (buck2) / linux-job](https://hud.pytorch.org/pr/pytorch/executorch/3646#25075703973) ([gh](https://github.com/pytorch/executorch/actions/runs/9119723068/job/25075703973)) (detected as infra flaky with no log or failing log classifier)

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This pull request was exported from Phabricator. Differential Revision: D57463419