Issue 6090 - Allow use of console to be seen during tests

test/_common.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ skipping 74 matching lines @@
       if (resourceName && resources.hasOwnProperty(resourceName))
         return {[resourceName]: resources[resourceName]};
 
       throw new Error(
         "Attempt to import unknown JavaScript module " + resource
       );
     },
     reportError(e) {}
   },
   console: {
-    log() {},
-    error() {}
+    log: console.log.bind(console),
+    error: console.error.bind(console)
   },
   navigator: {
   },
   onShutdown: {
     add() {}
   },
   URL
 };
 
 let knownModules = new Map();
@@ skipping 305 matching lines @@
     Cr,
     XMLHttpRequest,
     Date: {
       now: () => currentTime
     }
   };
 };
 
 console.warn = console.log;
 
+exports.silenceAssertionOutput = function(test, msg)
+{
+  let msgMatch = new RegExp(`^Error: ${msg}[\r\n]`);
+  let errorHandler = globals.console.error;
+  globals.console.error = s =>
+  {
+    if (!msgMatch.test(s))
+      errorHandler(s);
+  };
+  let result;
+  try
+  {
+    result = test();

[sergei, 2017/11/24 16:46:27]

There is no necessity in an additional variable, could you please just return
from the try-block?

[hub, 2017/11/24 18:17:56]

For some reason I believed that return would by-pass the finally.... 

Done

+  }
+  finally
+  {
+    globals.console.error = errorHandler;
+  }
+  return result;
+};
+
 exports.setupRandomResult = function()
 {
   let randomResult = 0.5;
   Object.defineProperty(this, "randomResult", {
     get: () => randomResult,
     set: value => randomResult = value
   });
 
   return {
     Math: Object.create(Math, {
       random: {
         value: () => randomResult
       }
     })
   };
 };
 
 exports.unexpectedError = function(error)
 {
   console.error(error);
   this.ok(false, "Unexpected error: " + error);
 };