Unfortunately it's not possible to automatically enable features
for `cargo test`. See https://github.com/rust-lang/cargo/issues/2911.
As a workaround to allow for using example domains for unit- and integration
tests, we introduce a new feature, `check_example_domains`, which is
disabled by default for normal users. The feature gets activated for the
integration test which checks that the example domain exclusion works as
expected.
