tests: use cli verify in end-to-end checks

[strimo378]

Motivation

• Consolidate end-to-end verification to the CLI verify path so tests exercise the same compile/build/verify flow used by users.
• Remove duplicated testbench build/run logic from tests and rely on the CLI's verification logic to compare against ONNX Runtime.
• Keep a small unit check for emitted initializer arrays while delegating correctness checks to the CLI.
• Simplify test code and reduce maintenance surface for end-to-end checks.

Description

• Replaced in-test compile/build/run logic with a helper that invokes the CLI onnx2c verify command in tests/test_endtoend_features.py and tests/test_endtoend_ops.py.
• Removed direct onnxruntime-based assertions from the tests and delegated numerical verification to the CLI verify command, while still asserting emitted initializer array presence/shape in the features test.
• Added _run_cli_verify helper and removed duplicated compiler invocation and temporary binary build steps from the operator tests.
• Cleaned up imports and test flow to use the unified CLI verification path.

Testing

• Ran pytest -q tests/test_endtoend_features.py tests/test_endtoend_ops.py.
• Result: 6 passed (all modified end-to-end tests succeeded).
• The CLI verify invocation is exercised by the tests and verified to complete successfully.
• No additional automated test failures were introduced by these changes.

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Codex Task