[ET] Fix pytest path for executorch export tests

[abhinaykukkadapu]

Fix unittest ci job failure due to export tests.

[pytorch-bot]

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🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/12826

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⏳ No Failures, 3 Pending

As of commit 60ef0c9 with merge base 7e82d00 ():
💚 Looks good so far! There are no failures yet. 💚

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