tests/test_ifrename_dynamic: Merge duplicate code into common functions

[bernhardkaindl]

tests/test_ifrename_dynamic: Merge duplicate code into two small common functions

The duplicated code was detected by a specialized tool, and it prepared merging them for me as well.

I reviewed the work and moved the common functions so that the change is easy to review.

$ git log -n1 -p | diffstat
  pyproject.toml                 |    2 +-
  tests/test_ifrename_dynamic.py |   44 ++++++++++++++------------------------------
  2 files changed, 15 insertions(+), 31 deletions(-)

The change to pyproject.toml is needed to make the max-columns for black less wide:
It stops black from create an overlong joined line in the context of the diff.

The plan would be anyways to make the code formatting less wide.
It's a good find that the columns (were 96) was too wide, and this improves it.