_PyCompile_CodeGen compiles nested function all the way to code object

[iritkatriel]

Bug report

The _PyCompile_CodeGen test utility is supposed to run only codegen and return an instruction sequence. However, it compiles nested functions all the way to code object. Need to teach it how to return the instruction sequence for the nested function.

Linked PRs

• gh-117901: Add option for compiler's codegen to save nested instruction sequences for introspection #118007