TDD should save the day though...
If a microservice is built well, to genuinely do *one* small job then it should lend itself to easy testing, programmatically - TDD / unit tests etc.
If devs make changes to their microservices they should have tests to run that prove the outputs of that service are the same for given input(s) etc.
Therefore it follows that full integration testing really isn't needed (I agree in theory) to release a new service version into a wider system.
The services just need to be small enough.
Would one retest an entire operating system when a new version of ping or ls was built?