./wpt run --install-fonts doesn't reliably work in macOS CI

[foolip]

@jgraham has had this problem in Gecko's CI, and it's happening in Azure Pipelines too:
https://dev.azure.com/foolip/wpt/_build/results?buildId=122&view=logs

The failure there manifested as:
▶ FAIL [expected PASS] /infrastructure/assumptions/ahem.html

This probably has something to do with when the ~/Library/Fonts/ directory is created and when it begins being monitored. Installing to /Library/Fonts works.

Note that --install-fonts works locally, at least for me.

[foolip]

It happened in https://dev.azure.com/foolip/wpt/_build/results?buildId=220&view=logs too, where I was making unrelated changes. "Run infrastructure/ tests (Chrome Dev)" failed on /infrastructure/assumptions/ahem.html.

[foolip]

Again, affected a real PR (#14178) this time: https://dev.azure.com/web-platform-tests/wpt/_build/results?buildId=1004&view=logs

[foolip]

web-platform-tests/rfcs#22 will make this not matter.