test(postproc): whether interpolate fills in existing values

WHY IS THIS CHANGE NEEDED?:
- need a simple test to see if gitlab visualises coverage

HOW DOES THE CHANGE SOLVE THE PROBLEM?:
- write a test for interpolate post-process; this test checks that
  interpolated values are as expected by leveraging a dummy signal that
  is a gradient.

REFERENCES:
- issue #46
- merge request !8