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

Stack from ghstack (oldest at bottom):

#4621
#4620
#4619
#4618
#4617
#4616
#4615
#4614
#4613
#4612
#4611
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#4609
-> #4608
#4607
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#4605

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: D60428952

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/4608

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As of commit 41f39f8d8619e3fd9b33893302f72f88aaf6bdf4 with merge base 5d151d05bd9da61110941247d78f23ba911e4d32 (): :green_heart: Looks good so far! There are no failures yet. :green_heart: