Account for the change in context isolation with Electron 12

See for more information https://github.com/twolfson/karma-electron/issues/50

src/karma.conf.js

@@ -36,7 +36,8 @@ module.exports = function (config) {
             nodeIntegration: true,
             nodeIntegrationInSubFrames: true,
             allowRunningInsecureContent: true,
-            enableRemoteModule: true
+            enableRemoteModule: true,
+            contextIsolation: false
           }
         }
       }