declare "allow_cache_wipe" marker in setup.cfg to avoid pytest warning

[ThomasWaldmann]

Undeclared markers trigger a pytest warning:

PytestUnknownMarkWarning: Unknown pytest.mark.allow_cache_wipe - is this a typo?

Backport of #5197

[codecov-commenter]

Codecov Report

Merging #5351 into 1.1-maint will decrease coverage by 0.70%.
The diff coverage is n/a.

@@              Coverage Diff              @@
##           1.1-maint    #5351      +/-   ##
=============================================
- Coverage      80.09%   79.39%   -0.71%     
=============================================
  Files             27       27              
  Lines          10396    10396              
  Branches        1829     1829              
=============================================
- Hits            8327     8254      -73     
- Misses          1555     1625      +70     
- Partials         514      517       +3     

Impacted Files	Coverage Δ
src/borg/xattr.py	59.61% <0.00%> (-22.60%)	⬇️
src/borg/platform/__init__.py	73.68% <0.00%> (-15.79%)	⬇️
src/borg/platform/base.py	79.43% <0.00%> (-3.74%)	⬇️
src/borg/archive.py	81.79% <0.00%> (-0.45%)	⬇️
src/borg/archiver.py	81.71% <0.00%> (-0.34%)	⬇️
src/borg/helpers.py	87.06% <0.00%> (-0.29%)	⬇️
src/borg/repository.py	85.23% <0.00%> (-0.20%)	⬇️

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