A simple javascript plugin architecture
Looks fine to me,

I would consider making extend a property of prototype though.

For nitpickings, I think

function Test() {
    this.tester = 'The instance was successfully created';
};


is preferred to 

test = function() {
    this.tester = 'The instance was successfully created';
};


This ways developers will know to use new when using test.
Please read Learning JavaScript Design Patterns for the cons of using the new keyword.

You probably want to store the entire plugin object provided to extend, not just keep init.

You probably want to throw an exception if the plugin does not have an init  property.

I am not sure if you realize that method1 and method2 are at the end of the script also "core methods"?