tests: net: lib: http_server: Check response

Building with clang, it warns:

tests/net/lib/http_server/core/src/main.c:1400:19: error: variable
'http1_header_capture_common_response' is not needed and will not be
emitted [-Werror,-Wunneeded-internal-declaration]
static const char http1_header_capture_common_response[]
                  ^                             = "HTTP/1.1 200\r\n"

Add a check to make sure the response actually matches the expected
response.

Signed-off-by: Tom Hughes <tomhughes@chromium.org>