replace "output_frequency" with "output_interval" for post-processing in tests

[mbkuhn]

Summary

Pull request type

Please check the type of change introduced:

• Bugfix
• Feature
• Code style update (formatting, renaming)
• Refactoring (no functional changes, no api changes)
• Build related changes
• Documentation content changes
• Other (please describe):

Checklist

The following is included:

• new unit-test(s)
• new regression test(s)
• documentation for new capability

This PR was tested by running:

• the unit tests
  • on GPU
  • on CPU
• the regression tests
  • on GPU
  • on CPU

Additional background

Issue Number:

[mbkuhn]

When you search the repo for "output_frequency", it is still used for ABL Stats and some other specific output types, such as actuators. It would be nice to consistently use "output_interval", but that will wait for another PR.