selftests/bpf: Modernize btf_dump test scaffolding

Refactor test_btf_dump_case() in order to:
  * use newer ASSERT_* macros instead of CHECK
  * drop the file-scope "duration" variable CHECK required

Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
Link: https://lore.kernel.org/r/20260828215207.3105313-3-ihor.solodrai@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>