selftests/bpf: Add connect_to_addr_str helper

selftests/bpf: Add connect_to_addr_str helper

Similar to connect_to_addr() helper for connecting to a server with the
given sockaddr_storage type address, this patch adds a new helper named
connect_to_addr_str() for connecting to a server with the given string
type address "addr_str", together with its "family" and "port" as other
parameters of connect_to_addr_str().

In connect_to_addr_str(), the parameters "family", "addr_str" and "port"
are used to create a sockaddr_storage type address "addr" by invoking
make_sockaddr(). Then pass this "addr" together with "addrlen", "type"
and "opts" to connect_to_addr().

Suggested-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
Link: https://lore.kernel.org/r/647e82170831558dbde132a7a3d86df660dba2c4.1721282219.git.tanggeliang@kylinos.cn
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>