Fix yac interpolation execute get test

Summary

Fixes the test routines for asynchronous opteration.

Instead of testing whether any interpolation is finished, all interpolations must be finished.

TODO

• How to test this?

Related Issues

none

Documentation

Checklist

• All code changes are covered by unit tests
• Code coverage did not deteriorate
• Code follows Style Guide
• Documentation updated, if necessary
• WIP: Manual running of configure/build jobs was successfull
• Update version info, if changes are worth mentioning (Develop section in doc/src/version_info.dox, add section if not yet included)