`L1HandlerTransaction::create_for_testing` should not be part of any non `test` binary

[tdelabro]

This is a test-only function. It should be behind a #[cfg(test] flag. Or if you want to have it available for other crates while testing, you may want to create a testing feature. But regular lib users, building regular production code, should not be exposed to this method.

[tdelabro]

A lot of other types have a similar method, but are implemented under crates/blockifier/src/test_utils/struct_impls.rs which is behind this flag #[cfg(any(feature = "testing", test))]. That's indeed the correct way to do it