executorch [executorch] Migrate runtime/executor tests to new namespace

Stack from ghstack (oldest at bottom):

#4621
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#4615
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The previous commit moved headers to the ::executorch::runtime namespace. This diff updates the tests for those changes. After this, these tests should not use ::torch::executor for the updated names.

Differential Revision: D60703363

Aug 09 '24 00:08 dbort

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:test_tube: See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/4616

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