Support for running tests

[UtkarshKunwar]

I use the flutter test test/ command quite frequently (In VSCode, I can just add an entry to the launch.json file and run it from there directly).
Right now, I am relying on executing the system command straight from neovim like :!flutter test test/ but the output is not exactly "pretty" as in green for passed or red for failed. This plugin's great, but if this feature could be added in the future as an enhancement, it would be great for development. Like a flutter.run.test command or something.
In the meantime, I'll try to do something from my side when I have time and see if I can contribute a PR.