selftests/bpf: Add tp_btf CO-RE reloc test for modules

Add another CO-RE relocation test for kernel module relocations. This time for
tp_btf with direct memory reads.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20201203204634.1325171-14-andrii@kernel.org