[Process] Added process synchronization to the incremental output tests

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.
minor #11565 [Process] Added process synchronization to the incremental output tests (webmozart)

This PR was merged into the 2.3 branch.

Discussion
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[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.

Commits
-------

6dd3946 [Process] Added process synchronization to the incremental output tests