Add UI tests

[hoxbro]

Mostly copied from Panel setup

[codecov-commenter]

Codecov Report

Merging #5764 (bcfdbac) into main (14d7d9d) will decrease coverage by 0.02%.
The diff coverage is 96.87%.

@@            Coverage Diff             @@
##             main    #5764      +/-   ##
==========================================
- Coverage   88.18%   88.16%   -0.02%     
==========================================
  Files         306      307       +1     
  Lines       62739    62761      +22     
==========================================
+ Hits        55326    55336      +10     
- Misses       7413     7425      +12     

Flag	Coverage Δ
ui-tests	22.40% <90.62%> (?)

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Impacted Files	Coverage Δ
holoviews/tests/ui/bokeh/test_layout.py	94.11% <94.11%> (ø)
holoviews/tests/conftest.py	100.00% <100.00%> (ø)

... and 7 files with indirect coverage changes

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[maximlt]

That's a good start, it's just missing some HoloViews tests :D Do you intend to add some in this PR directly?

I've left some comments. I'm also going to add something that will fall right into the nice to have category and isn't a blocker for this PR. pytest's output shows all the skipped tests which isn't super useful:

Not showing that doesn't seem to be super straightforward (pytest-dev/pytest#3730).

[hoxbro]

Not showing that doesn't seem to be super straightforward

Looks good but we still want to show skipped UI tests, correct?