change test behaviour a little

reverting an earlier change that always did a testrun, also made 'test'
task more robust against unconfigured karma
add cli switch to disable test server

this adds support for the following use case:

As a developer i usually want to run `grunt dev`. Most of the time, I
want a test server to be started, but sometimes I want to skip that.

This can now be done using: `grunt dev` vs. `grunt dev --no-tests`.