bpf: selftests: Move xfrm tunnel test to test_progs

bpf: selftests: Move xfrm tunnel test to test_progs

test_progs is better than a shell script b/c C is a bit easier to
maintain than shell. Also it's easier to use new infra like memory
mapped global variables from C via bpf skeleton.

Co-developed-by: Antony Antony <antony.antony@secunet.com>
Signed-off-by: Antony Antony <antony.antony@secunet.com>
Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Link: https://lore.kernel.org/r/a350db9e08520c64544562d88ec005a039124d9b.1702593901.git.dxu@dxuuu.xyz
Signed-off-by: Alexei Starovoitov <ast@kernel.org>