BF: do use OBSCURE_FILENAME instead of hardcoded unicode

[yarikoptic]

OBSCURE_UNICODE should be used since otherwise tests would
just keep crashing while running on file systems/environment
which do not support those fancy characters.

Initial intention here was to address some Windows fails on Vista and
other elderly systems. #2929
If tests pass on current CI I would assume that we are past that
stone age.

Closes a part of #5942. Wanted first to see if there are side-effects

[codecov]

Codecov Report

Merging #5944 (44aa5ab) into maint (dfdd352) will decrease coverage by 6.56%.
The diff coverage is 100.00%.

❗ Current head 44aa5ab differs from pull request most recent head 0ceb5ff. Consider uploading reports for the commit 0ceb5ff to get more accurate results

@@            Coverage Diff             @@
##            maint    #5944      +/-   ##
==========================================
- Coverage   90.28%   83.72%   -6.57%     
==========================================
  Files         300      297       -3     
  Lines       42516    41375    -1141     
==========================================
- Hits        38387    34642    -3745     
- Misses       4129     6733    +2604     

Impacted Files	Coverage Δ
datalad/support/tests/test_globbedpaths.py	100.00% <100.00%> (ø)
datalad/support/tests/test_network.py	98.60% <100.00%> (-0.01%)	⬇️
datalad/utils.py	79.64% <100.00%> (-5.46%)	⬇️
datalad/support/tests/utils.py	0.00% <0.00%> (-100.00%)	⬇️
datalad/metadata/extractors/tests/test_audio.py	20.00% <0.00%> (-80.00%)	⬇️
datalad/metadata/extractors/xmp.py	12.96% <0.00%> (-79.63%)	⬇️
datalad/metadata/extractors/exif.py	18.75% <0.00%> (-78.13%)	⬇️
datalad/metadata/extractors/image.py	19.35% <0.00%> (-77.42%)	⬇️
datalad/metadata/extractors/audio.py	20.00% <0.00%> (-77.15%)	⬇️
datalad/metadata/extractors/tests/test_exif.py	25.00% <0.00%> (-75.00%)	⬇️
... and 197 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update dfdd352...0ceb5ff. Read the comment docs.

[yarikoptic]

besides codecov all green, good, so I assume that appveyor is our gold standard for minimal version of windows to be compatible and will proceed fixing other similar failures in the same fashion.

[yarikoptic]

Windows doesn't like my questionable fix

TypeError: bytes args is not allowed on Windows

[yarikoptic]

Extracted a more "core" change to #5961 so this PR now concerns only with tests.

[adswa]

I can't spot anything wrong with this PR, looks sensible to my not-so-expert eyes :)

[yarikoptic]

Thank you @adswa for feedback on all PRs staged for the release. Let's cut out the release by merging this PR, adding the label

[yarikoptic]

releasing failed (#5972 ), taking off the label since we might just want to retry