selftests: bpf: add tests for dead code removal

Add tests for newly added dead code elimination.  Both verifier
and BTF tests are added.  BTF test infrastructure has to be
extended to be able to account for line info which is eliminated
during dead code removal.

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Acked-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>