tests: rename C testbench to end-to-end test file

[strimo378]

Motivation

• Rename the C testbench execution test to clearly indicate it is an end-to-end test.
• Keep golden codegen checks separated from environment-dependent compilation and runtime comparisons.
• Make test file naming consistent with the test behavior to improve discoverability.

Description

• Renamed tests/test_c_testbench.py to tests/test_endtoend.py and added the new file to the repo.
• Added a header note to tests/test_golden.py indicating it contains only golden codegen tests.
• The end-to-end test compiles emitted C using CompilerOptions(template_dir=Path("templates"), emit_testbench=True), runs the binary, and skips when no C compiler is available.

Testing

• Ran pytest -n auto -q against the codebase.
• The test suite completed successfully with 12 passed.

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Codex Task