TEST: Avoid using just 'example.com' - test_cmp_http

TEST: Avoid using just 'example.com'  - test_cmp_http

We have reports that some are using example.com in their /etc/hosts
for testing purposes, so we can't necessarily assume that those will
fail.

We fix it by using "random" hosts in that domain.

Fixes #15395

Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/15398)