Welcome to KolibriOS.
I will try to answer parts that I'm aware of at the moment.
GSoC idea page suggests that "submit a small bug fix/improvement to SVN".
Can I get some test task to work upon ?
Your potential mentor (or another developer) will provide you a test task (usually related to the GSoC idea you want to work on). So I guess best is to wait it out and understand the code mean while and set up Kolibri till then.
From where I can start understanding KolibriOS code ?
Any documentation on workflow would be great.
IMO, Setting up and running Kolibri should be useful for this. You should also try to read the source code for example programs and other programs.
You can go through the wiki pages to read some articles about development , understand the basic binary header structure and so on.
What is best way of installing KolibriOS from developer's perspective ? I mean I want to develop and run KolibriOS side by side. Any specific emulator for that ?
I am using ubuntu 14.04.
Different developers prefer different options. My personal setup includes qemu + latest kolibri image right now . In the past I have used VirtualBox for it as well as installed it on a desktop machine alongside a Windows installation. Again, you should be able to find these instructions on the Wiki itself.
In case you have other queries / questions, I think you should go through the wiki as it should answer most questions. In case it doesn't feel free to post back or post on IRC / Forum Chat.
Most importantly, Have fun!
Check out the Netsurf
Web Browser for KolibriOS.
Read the wiki and happy hacking with KolibriOS!