I know a lot of folks using .dev for locally running "development" instances of websites/apps. Maybe .local is now a better choice.
test-service-1.zeus.barrera.io dummy-server.zeus.barrera.i
Etc. No need to create new (possible conflicting, definitely not future proof) TLDs.
my-test-thing-a.dev 127.0.0.1
my-test-thing-b.dev 127.0.0.1
my-test-thing-c.dev 127.0.0.1
Buying an actual registered domain for that sounds like it would be even more confusing than using an unregistered TLD because it makes it hard to tell which hostnames are expected to work across machines and which are strictly "works on my machine".