selftests/bpf: Revert CO-RE removal in test_ksyms_weak.

The commit 087cba79

 ("selftests/bpf: Add weak/typeless ksym test for light skeleton")
added test_ksyms_weak to light skeleton testing, but remove CO-RE access.
Revert that part of commit, since light skeleton can use CO-RE in the kernel.

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20211201181040.23337-17-alexei.starovoitov@gmail.com