test-env-util: Verify that \r is disallowed in env var values

test-env-util: Verify that \r is disallowed in env var values

This adds tests to make sure that basic/env-util considers environment
variables containing \r characters invalid, and that it removes such
variables during environment cleanup in strv_env_clean*().

test-env-util has not verified this behaviour before.

As \r characters can be used to hide information, disallowing them
helps with systemd's security barrier role, even when the \r
character comes as part of a DOS style (\r\n) line ending.

Prompted-by: https://github.com/systemd/systemd/issues/17378