tests: dns_name -> server_name.

[cpu]

This commit updates the tests/client.c program to fix a stale reference to the now renamed dns_name field of the rustls_verify_server_cert_params struct. This field is now called server_name to reflect that it may contain an IP address.

This fixes the following build error:

Run make CC=clang PROFILE=release test
mkdir -p target
clang -o target/client.o -c tests/client.c -Werror -Wall -Wextra -Wpedantic -g -I src/ -fsanitize=address -fsanitize=undefined -O3
tests/client.c:37[6](https://github.com/rustls/rustls-ffi/actions/runs/4502338870/jobs/7924017944?pr=303#step:4:7):24: error: no member named 'dns_name' in 'struct rustls_verify_server_cert_params'
          (int)params->dns_name.len,
               ~~~~~~  ^
tests/client.c:3[7](https://github.com/rustls/rustls-ffi/actions/runs/4502338870/jobs/7924017944?pr=303#step:4:8)7:19: error: no member named 'dns_name' in 'struct rustls_verify_server_cert_params'
          params->dns_name.data);
          ~~~~~~  ^
2 errors generated.
Makefile:4[8](https://github.com/rustls/rustls-ffi/actions/runs/4502338870/jobs/7924017944?pr=303#step:4:9): recipe for target 'target/client.o' failed
make: *** [target/client.o] Error 1
Error: Process completed with exit code 2.

[cpu]

With this and #303 combined I think we'll be building 🟢 finally 🎉

[jsha]

Thanks!