Remove -Wno-* from test_cli.py
The test runner adds `-Wno-unused-variable` and `-Wno-null-arithmetic` to the c++ compiler opts.

Nim also expects no unused variables, so it is unusual that C++ might have unused variables for the same input, and likely means the C++ transpiler is emitting this.  However if there are unused variables in the cases, those should be removed.

Fixing null-arithmetic could be more difficult.  If it is hard to fix, the intentional null-arithmetic should be moved into a dedicated test, so that the very broad `coverage.py` doesnt need extra flags.