iOS CoreTest coverage for libuv's dns resolver component

[dennycd]

Add libuv-based unit test on iOS core test for DNS resolver. This make sure the API that core uses can work. Unit test logic can be sampled from a core subset from

• https://github.com/libuv/libuv/blob/v1.x/test/test-getaddrinfo.c
• https://github.com/libuv/libuv/blob/v1.x/test/test-gethostname.c
• https://github.com/libuv/libuv/blob/v1.x/test/test-getnameinfo.c
• https://github.com/libuv/libuv/blob/v1.x/test/test-getsockname.c

Libuv's DNS utility API

• http://docs.libuv.org/en/v1.x/dns.html

[dennycd]

Proof-of-concept commit porting test-getaddrinfo.c with ios_unit_test setup via bazel

https://github.com/dennycd/grpc/commit/ef193cefba6d54d7cf5afad801d53e2fc020c7a0

[dennycd]

Pending for grpc #27513 to land