docs(chromium-web-tests): add debugging tips (#11684)

lighthouse-core/test/chromium-web-tests/README.md

@@ -14,6 +14,13 @@ yarn test-devtools
 yarn update:test-devtools
 ```
 
+### Debugging
+
+* Want logs from Lighthouse? Add `log.log('status', '**** hello test output ' + JSON.stringify({obj}));` which will be visible in the `lighthouse-successful-run.js` output thanks to `LighthouseTestRunner.addStatusListener`.
+* Want logs from test files? Adding these flags to the invocation `yarn test-devtools --driver-logging --no-retry-failures` will print to terminal.
+* Want logs from the inspected page? Add `testRunner.setDumpConsoleMessages(true);` to a test file. (also, [beware](https://source.chromium.org/chromium/chromium/src/+/master:content/web_test/renderer/web_view_test_proxy.cc;l=125-129;drc=437e5d9a05535b9e2cd7b983f78b23ebc3d92b3f) w/e this is about)
+
+
 ## How it works
 
 Normally, running these webtests requires a full Chromium checkout. However, that takes much too long, so it wouldn't be feasible for daily development or CI. Instead, we: