Report: make sure warnings within Passed Audits are visible

[paulirish]

Breaking this out of the discussion here:
#5270 (comment)

Current plan is to do a little block of warnings following the "Passed Audits" section:

I'll likely be landing this after the toplevel->auditlevel & visual changes in #5270

cc @hwikyounglee

[brendankenny]

Also need to update and enable the test skipped here:

lighthouse/lighthouse-core/test/report/html/renderer/category-renderer-test.js

Line 195 in 0f680b3

it.skip('renders the failed audits grouped by group', () => {

(or replace it with a new one or whatever...either way the skipped one just needs to go :)