[BEAM-14471] Fix PytestUnknownMarkingWarning

[Abacn]

• Fix PytestUnknownMarkingWarning seen in pytest result

• Also fix a backslash warning apearing nearby in log

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Codecov Report

Merging #17825 (45a80f7) into master (937e22f) will increase coverage by 0.00%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #17825   +/-   ##
=======================================
  Coverage   74.08%   74.09%           
=======================================
  Files         697      697           
  Lines       91986    91986           
=======================================
+ Hits        68146    68153    +7     
+ Misses      22591    22584    -7     
  Partials     1249     1249           

Flag	Coverage Δ
python	83.75% <ø> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...eam/runners/portability/fn_api_runner/execution.py	92.44% <0.00%> (-0.65%)	⬇️
...ks/python/apache_beam/runners/worker/sdk_worker.py	88.94% <0.00%> (-0.16%)	⬇️
...hon/apache_beam/runners/worker/bundle_processor.py	93.54% <0.00%> (+0.12%)	⬆️
sdks/python/apache_beam/transforms/util.py	96.22% <0.00%> (+0.15%)	⬆️
...che_beam/runners/interactive/interactive_runner.py	91.39% <0.00%> (+1.32%)	⬆️
...ks/python/apache_beam/runners/worker/data_plane.py	89.26% <0.00%> (+1.69%)	⬆️
.../python/apache_beam/testing/test_stream_service.py	92.85% <0.00%> (+4.76%)	⬆️

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[Abacn]

R: @chamikaramj
R: @ihji

[chamikaramj]

LGTM. Thanks.