selftests/bpf: Test "incremental" btf_dump in C format

Add test validating that btf_dump works fine with BTFs that are modified and
incrementally generated.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200929232843.1249318-5-andriin@fb.com