test: add pytest.mark.issue marker

[hoxbro]

Description

Saw this in another repo. Though, not entirely sure if I we should do it...

[codecov]

Codecov Report

❌ Patch coverage is 92.45283% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.33%. Comparing base (7a2b1d9) to head (8cc2b10).
⚠️ Report is 33 commits behind head on main.

Files with missing lines	Patch %	Lines
holoviews/tests/conftest.py	55.55%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6942   +/-   ##
=======================================
  Coverage   89.33%   89.33%           
=======================================
  Files         344      344           
  Lines       74608    74659   +51     
=======================================
+ Hits        66650    66700   +50     
- Misses       7958     7959    +1     

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