nixos/tests/acme: use *.test domains

Shimming out the Let's Encrypt domain name to reuse client configuration
doesn't work properly (Pebble uses different endpoint URL formats), is
recommended against by upstream,[1] and is unnecessary now that the ACME
module supports specifying an ACME server. This commit changes the tests
to use the domain name acme.test instead, and renames the letsencrypt
node to acme to reflect that it has nothing to do with the ACME server
that Let's Encrypt runs. The imports are renamed for clarity:

* nixos/tests/common/{letsencrypt => acme}/{common.nix => client}
* nixos/tests/common/{letsencrypt => acme}/{default.nix => server}

The test's other domain names are also adjusted to use *.test for
consistency (and to avoid misuse of non-reserved domain names such
as standalone.com).

[1] https://github.com/letsencrypt/pebble/issues/283#issuecomment-545123242



Co-authored-by: Yegor Timoshenko <yegortimoshenko@riseup.net>