test: Test safe GetElementCount and GetByteSize APIs at common and core repos

[yinggeh]

What does the PR do?

• Test new GetElementCount and GetByteSize added to common and core repos.
• Reuse backend_tensor_size_test.cc and rename to tensor_size_test.cc.

Checklist

• PR title reflects the change and is of format <commit_type>: <Title>
• Changes are described in the pull request.
• Related issues are referenced.
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• Added test plan and verified test passes.
• Verified that the PR passes existing CI.
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Commit Type:

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• test

Related PRs:

triton-inference-server/core#475
triton-inference-server/common#150

Where should the reviewer start?

Test plan:

• CI Pipeline ID:
  45921643

Caveats:

Background

Related Issues: (use one of the action keywords Closes / Fixes / Resolves / Relates to)

• closes GitHub issue: #xxx