REF/TST: Add more pytest idiom to test_to_html.py

[simonjayhawkins]

No description provided.

[jreback]

lgtm. ping on green.

[codecov]

Codecov Report

Merging #24609 into master will increase coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #24609      +/-   ##
==========================================
+ Coverage   92.38%   92.38%   +<.01%     
==========================================
  Files         166      166              
  Lines       52490    52490              
==========================================
+ Hits        48493    48494       +1     
+ Misses       3997     3996       -1

Flag	Coverage Δ
#multiple	90.81% <ø> (ø)	⬆️
#single	43.05% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
pandas/util/testing.py	88.09% <0%> (+0.09%)	⬆️

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[codecov]

Codecov Report

Merging #24609 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #24609      +/-   ##
==========================================
- Coverage   92.38%   92.37%   -0.01%     
==========================================
  Files         166      166              
  Lines       52490    52471      -19     
==========================================
- Hits        48493    48471      -22     
- Misses       3997     4000       +3

Flag	Coverage Δ
#multiple	90.8% <ø> (-0.02%)	⬇️
#single	43.06% <ø> (+0.01%)	⬆️

Impacted Files	Coverage Δ
pandas/core/internals/blocks.py	94.21% <0%> (-0.33%)	⬇️
pandas/core/indexes/datetimelike.py	98.53% <0%> (-0.31%)	⬇️
pandas/core/nanops.py	94.36% <0%> (-0.19%)	⬇️
pandas/core/arrays/categorical.py	95.41% <0%> (-0.06%)	⬇️
pandas/core/indexes/category.py	98.61% <0%> (-0.04%)	⬇️
pandas/core/internals/__init__.py	100% <0%> (ø)	⬆️
pandas/core/series.py	93.56% <0%> (+0.02%)	⬆️
pandas/core/internals/managers.py	96.05% <0%> (+0.08%)	⬆️
pandas/util/testing.py	88.09% <0%> (+0.09%)	⬆️
pandas/core/arrays/timedeltas.py	88.18% <0%> (+0.19%)	⬆️
... and 1 more

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[jreback]

pandas/tests/io/formats/test_to_html.py(244,): error :  Found unwanted pattern:     with pytest.warns(FutureWarning, match=msg)
Bash exited with code '1'.

use assert_produces_warning

[jreback]

thanks!