tests: add test for symbol length

[nbdd0121]

__pinned_init or __init functions are natural inline boundaries. Things within them calling into helper functions should be inlined, and wrapping functions that forwarding call to them should be, too. But these functions themselves can contain complex initialization, so it's best to leave it to the compilers to decide if they're inlineable.

As a result, these symbols do show up in the symbol table when they're not inlined. Linux kernel has 512 byte symbol length limit, so we need to keep them small. Add a test to ensure that any growth of them are being considered and not accidental.

[nbdd0121]

An alternative is to mark them as #[inline(always)] and have the closure (generated by pin_init!() macro) themselves being where the bulk of the code lives.

Actually, that's probably better, because if they're created inside a new function, then the generated symbol name is just going to be new::{closure#0} after mangling. E.g.

<kernel::scatterlist::SGTable<kernel::scatterlist::Owned<kernel::alloc::kvec::Vec<u8, kernel::alloc::allocator::Vmalloc>>>>::new::{closure#0}

That's more clear than <pin_init::__internal::InitClosure<..., ..., ...> as pin_init::PinInit<...>>::__pinned_init

[nbdd0121]

The type name of the initializer also shows up in generic code that uses it, too, e.g. InPlaceInit. So we still want shorter symbol names for these.