selftests/bpf: Use start_test in test_dctcp in bpf_tcp_ca

The "if (sk_stg_map)" block in do_test() is only used by test_dctcp(),
it makes sense to move it from do_test() into test_dctcp(). Then
do_test() can be used by other tests except test_dctcp().

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/9938916627b9810c877e5c03a621bc0ba5acf5c5.1717054461.git.tanggeliang@kylinos.cn