Refactor AddTest()-calls to OgsTest() (part 1)

Lars Bilkerequested to merge
bilke/ogs:ogstest into master

Another take on !5699 (closed).

This is a first batch of changes: only simpler conversions.

  1. Tests covering your feature were added?
  2. Any new feature or behaviour change was documented?
Edited by Dmitri Naumov

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