[Process] Added process synchronization to the incremental output tests

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        | -

The tests currently fail from time to time if the executing machine is under
heavy load. This leads to false negatives on Travis CI.

A side effect of the change is that the tests are much faster now.

This PR is WIP as I would like to do similar changes for other process tests.
We could, but that increases server load unnecessarily if one process has to wait for the other one.