fix failing build on hosts with wildcard dns (fixes #1230)

[Jennes]

on hosts which have wildcard dns domains in their local domain search list, builds failed because "nosuch.host" could actually be resolved.

Since ".host" isn't a recommended TLD by RFC2606, we use ".invalid" now. And since this is not enough to fix the problem, we use now absoulte domain names (having a '.' at the end)

Fixes #

Requirements

All new code should be covered with tests, documentation should be updated. CI should pass.

Description of the Change

Checklist

• [ ] unit-test added (if change is algorithm)
• [ ] functional test added/updated (if change is functional)
• [ ] man page updated (if applicable)
• [ ] bash completion updated (if applicable)
• [ ] documentation updated
• [ ] author name in AUTHORS

[Jennes]

On my local machines the tests work fine (with or without the wildcard domains). So I guess there is something wrong with DNS resolution on the Github runner. But what would be a decent test which also works within Github?