selftests/bpf: Replace CHECK with ASSERT macros for ksyms test

Replace CHECK with ASSERT macros for ksyms tests.
This test failed earlier with clang lto kernel, but the
issue is gone with latest code base. But replacing
CHECK with ASSERT still improves code as ASSERT is
preferred in selftests.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20240326041448.1197812-1-yonghong.song@linux.dev
Signed-off-by: Alexei Starovoitov <ast@kernel.org>