selftests/bpf: Add dynptr helper tests

First test that we allow overwriting dynptr slots and reinitializing
them in unreferenced case, and disallow overwriting for referenced case.
Include tests to ensure slices obtained from destroyed dynptrs are being
invalidated on their destruction. The destruction needs to be scoped, as
in slices of dynptr A should not be invalidated when dynptr B is
destroyed. Next, test that MEM_UNINIT doesn't allow writing dynptr stack
slots.

Acked-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20230121002241.2113993-13-memxor@gmail.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>