chore(flakiness): Flakiness Dashboard fixes (#4788)

- fix `FLAKINESS_DASHBOARD_BUILD_URL` to point to a task instead of a build
- do not pretty-print `dashboard.json` when serializing flakiness results
- filter out 'COVERAGE' test(s) so that they don't add up to `dashboard.json` payload. These are useless
- validate certain important options of flakiness dashboard
- more logging to STDOUT to actually say which repo and what branch is getting used
- enhance commit message with a build URL
- use a more compact format for JSON. For 100 runs of 700 tests it yields 21MB json instead of 23MB.
- bump default builds number to 100

test/utils.js

@@ -191,6 +191,10 @@ const utils = module.exports = {
     });
 
     testRunner.on('testfinished', test => {
+      // Do not report tests from COVERAGE testsuite.
+      // They don't bring much value to us.
+      if (test.fullName.startsWith('COVERAGE'))
+        return;
       const testpath = test.location.filePath.substring(utils.projectRoot().length);
       const url = `https://github.com/GoogleChrome/puppeteer/blob/${sha}/${testpath}#L${test.location.lineNumber}`;
       dashboard.reportTestResult({