selftests/bpf: Add start_server_str helper

selftests/bpf: Add start_server_str helper

It's a tech debt that start_server() does not take the "opts" argument.
It's pretty handy to have start_server() as a helper that takes string
address.

So this patch creates a new helper start_server_str(). Then start_server()
can be a wrapper of it.

Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Link: https://lore.kernel.org/r/606e6cfd7e1aff8bc51ede49862eed0802e52170.1716638248.git.tanggeliang@kylinos.cn
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>