resolv: Add tests for getaddrinfo returning EAI_AGAIN [BZ #16849]

resolv: Add tests for getaddrinfo returning EAI_AGAIN [BZ #16849]

This patch adds two tests that verify correct behavior of getaddrinfo
when DNS resolution fails with a temporary error. Both tests ensure
that getaddrinfo returns EAI_AGAIN in cases where no valid address can
be resolved due to network or resolver failure.

* tst-getaddrinfo-eai-again.c
  Runs inside the glibc test-container without any DNS server
  configured. The test performs queries using AF_INET, AF_INET6,
  and AF_UNSPEC and verifies that getaddrinfo returns EAI_AGAIN
  when resolution fails.

* tst-getaddrinfo-eai-again-timeout.c
  Runs outside of the container but uses the resolv_test framework
  to simulate network failures. The test covers two failure modes:
  - No response from the server (resolv_response_drop)
  - Zero-length reply from the server
  In both cases, getaddrinfo is expected to return EAI_AGAIN.

Reviewed-by: Florian Weimer <fweimer@redhat.com>