Split out HierarchicalAllocator tests (#386)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/386

A following diff will modify HierarchicalAllocator and add more tests.

Clean up the headers and deps of `executor_test` while I'm here.
ghstack-source-id: 201071494
exported-using-ghexport

Reviewed By: tarun292

Differential Revision: D49344930

fbshipit-source-id: 511cc0e35693626e549c3ca6f5f2513c2cd71f46