bpf: Refactor BTF encoding macro to test_btf.h

Refactor common BTF encoding macros for other tests to use.
The libbpf may reuse some of them in the future  which requires
some more thoughts before publishing as a libbpf API.

Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>