Tree UI for test directory structure rollup

Current table headings:
* Test Case (path)
* stdout
* stderr
* Result

Rollup table UI to directory structure and subtotals. Dynamic dropdowns to expand on groupings.

Toggle switch to go back to current mode.

Compare with eg, https://wpt.fyi/clear-site-data and https://wpt.fyi/clear-site-data/navigation.https.html