Use feature instead of exporting the crate for testing with hidden doc

[xieyuschen]

Currently, iceoryx exposes an API for testing purpose and hides its docs. I think a more reasonable approach is define a feature called iox2-test for it so we can ensure it won't be involved inside users code except the explicit feature setting up. A testing feature is needed because the default test doesn't support cross package usage.

iceoryx2/iceoryx2-bb/elementary/src/bump_allocator.rs

Lines 17 to 20 in 558c727

	/// Simple BumpAllocator for testing purposes. Do not use this in production. If you are looking
	/// for a production ready BumpAllocator use the one from iceoryx2_bb_memory::bump_allocator
	#[doc(hidden)]
	pub struct BumpAllocator {