feat(test): enable dumpio in tests #2610

This patch allows logging the output of the Chromium process to be enabled in tests by passing in the environment variable `DUMPIO=true`.

Additionally, the `stderr` of the Chromium process will always be logged in the the "Output" section of failing page tests.

utils/testrunner/Reporter.js

@@ -83,6 +83,10 @@ class Reporter {
             console.log(stack.join('\n'));
           }
         }
+        if (test.output) {
+          console.log('  Output:');
+          console.log(test.output.split('\n').map(line => '    ' + line).join('\n'));
+        }
         console.log('');
       }
     }