[nlpo3-python]: Allow unreachable code during test

[bact]

Set RUSTFLAGS = -A unreachable_code environment for cargo test to suppressed "unreachable code" errors/warnings

• They are warnings, still able to successfully built, in my local environment

  stable-x86_64-apple-darwin (default)
  rustc 1.82.0 (f6e511eec 2024-10-15)

• The same warnings are converted to errors in "cargo test" in the CI and made the build/test failed (see this run for example)

I'm not sure if this flag setting is a good practice.

Will the code still not able to build elsewhere?