test(format/html): set up test infrastructure

[dyc3]

Summary

This PR sets up everything necessary to start adding snapshot tests for the html formatter.

Test Plan

The example snapshot test works and is evaluated correctly, and importing prettier's html tests also work (I just haven't done that to make reviewing this easier).

[github-actions]

Parser conformance results on

js/262

Test result	main count	This PR count	Difference
Total	48511	48511	0
Passed	47320	47320	0
Failed	1191	1191	0
Panics	0	0	0
Coverage	97.54%	97.54%	0.00%

jsx/babel

Test result	main count	This PR count	Difference
Total	40	40	0
Passed	37	37	0
Failed	3	3	0
Panics	0	0	0
Coverage	92.50%	92.50%	0.00%

symbols/microsoft

Test result	main count	This PR count	Difference
Total	6569	6569	0
Passed	2202	2202	0
Failed	4367	4367	0
Panics	0	0	0
Coverage	33.52%	33.52%	0.00%

ts/babel

Test result	main count	This PR count	Difference
Total	671	671	0
Passed	599	599	0
Failed	72	72	0
Panics	0	0	0
Coverage	89.27%	89.27%	0.00%

ts/microsoft

Test result	main count	This PR count	Difference
Total	18420	18420	0
Passed	14102	14102	0
Failed	4318	4318	0
Panics	0	0	0
Coverage	76.56%	76.56%	0.00%

[codspeed-hq]

CodSpeed Performance Report

Merging #3798 will not alter performance

_{Comparing 09-05-test_format_html_set_up_test_infrastructure (1a54307) with main (a66e450)}

Summary

✅ 107 untouched benchmarks