tests: rename end-to-end operator suite

[strimo378]

Motivation

• Rename the end-to-end operator test module to better reflect that it contains operator-specific tests and to match the project naming conventions.
• Make the operator test suite easier to locate and identify as test_endtoend_ops.py for clarity in test runs.
• Keep the test behavior and generated testbench checks unchanged while only moving the file.

Description

• Rename tests/test_endtoend.py to tests/test_endtoend_ops.py preserving the full test contents.
• The renamed file still provides _make_operator_model, _compile_and_run_testbench, the OPERATOR_CASES array, and the parameterized test_operator_c_testbench_matches_onnxruntime test.
• No functional changes were made to test logic, model construction, or the C compiler invocation.

Testing

• Ran pytest -q tests/test_endtoend_ops.py to validate the renamed test module.
• All parameterized cases succeeded and the run reported 4 passed.

---

Codex Task