bpf: btf: Some test_btf clean up

This patch adds a CHECK() macro for condition checking
and error report purpose.  Something similar to test_progs.c

It also counts the number of tests passed/skipped/failed and
print them at the end of the test run.

Signed-off-by: Martin KaFai Lau <[email protected]>
Acked-by: Alexei Starovoitov <[email protected]>
Acked-by: Song Liu <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>