Move `TESTING_TARGET` and `make_ir_from_items` into `ir_testing.rs`.
This helps to keep test-only things out of `ir.rs` (although `ir.rs` still contains
some test-only things like `make_ir_from_parts`).
PiperOrigin-RevId: 445138017
Extremely experimental interop tooling for C++ and Rust.
Please don‘t use, this is an experiment and we don’t yet know where will it take us. There will be breaking changes without warning. Unfortunately, we can't take contributions at this point.