bpf: migrate ebpf ld_abs/ld_ind tests to test_verifier

Remove all eBPF tests involving LD_ABS/LD_IND from test_bpf.ko. Reason
is that the eBPF tests from test_bpf module do not go via BPF verifier
and therefore any instruction rewrites from verifier cannot take place.

Therefore, move them into test_verifier which runs out of user space,
so that verfier can rewrite LD_ABS/LD_IND internally in upcoming patches.
It will have the same effect since runtime tests are also performed from
there. This also allows to finally unexport bpf_skb_vlan_{push,pop}_proto
and keep it internal to core kernel.

Additionally, also add further cBPF LD_ABS/LD_IND test coverage into
test_bpf.ko suite.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>