selftests/bpf: Extend distilled BTF tests to cover BTF relocation

Ensure relocated BTF looks as expected; in this case identical to
original split BTF, with a few duplicate anonymous types added to
split BTF by the relocation process.  Also add relocation tests
for edge cases like missing type in base BTF and multiple types
of the same name.

Signed-off-by: Alan Maguire <alan.maguire@oracle.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/bpf/20240613095014.357981-5-alan.maguire@oracle.com