feat log to browser console in JSUnit in debug mode

Currently, the SilentArrayAppender is the appender used in our JSUnit.
This means that things like `$logError` or `$logWarn` will not displayed in Firebug console.

However, the appender is changed from `DefaultAppender` in the middle of the execution, which results in some logs actually being available in the browser console, but the rest (after changing appender) not.

For debugging purposes, it's useful to see the errors and warnings directly
in the browser when running the test.