tests-ui.yml github action workflow fails unexpectedly intermittently

[jgravois]

lately i've noticed more intermittent flakiness in tests.ui.yml that i've been able to workaround via manually rerunning the workflow.

i took a quick look at the lighthouse docs and noticed that our attempt to trigger 3 runs isn't being honored, likely because the configuration syntax changed after @machikoyasuda last touched this back in #202.

benefits/.github/lighthouserc.json

Lines 2 to 3 in c16c025

	"ci": {
	"numberOfRuns": 3,

Our options

1. start triggering 3 runs again (as proposed in fix: set lighthouse numberOfRuns correctly to ensure gh actions honors it #4039).

to be perfectly honest its possible that one failure out of three will still fail the entire job, but it'd be a cheap test to just merge it and 'wait and see'. the only drawback being that the total runtime increases ~1 minute.

2. continue to run lighthouse only once, remove the unused configuration setting, and tolerate/ignore the intermittent errors

3. put some effort into diagnosing why calls to / and /help in the container are failing more often than they used to. is it a timing thing? an external dependency that doesn't load consistently?

Acceptance Criteria

• tests-ui.yml passes in gh action runs associated with unrelated PRs 100% of the time.
• our lighthouse config file contains no ignored/invalid declarations