app.js

// this would actually be in a beforeEach or something
// just haven't done that yet

var angular = require('angular');

var app = angular.module('app', [])
    .directive('someDirective', require('./myDirective'));


module.exports = app;

myDirective.js

module.exports = function () {
	return function (scope, elem) {
		elem.addClass('fizzbuzz');
	};
}

test.js

var test = require('tape'),
	proxyquire = require('proxyquire').noCallThru(),
	angular = require('angular-node')(),
	app = proxyquire('./app', {'angular': angular});
	inject = angular.injector(['ng', 'app']).invoke;

test('someDirective', function (t) {
	var el;

	inject(function($rootScope, $compile, $document) {
		el = angular.element('<div some-directive>{{ 7 + 5 }}</div>');
		el = $compile(el)($rootScope);
		$rootScope.$digest();
	})

	t.true(el.hasClass('fizzbuzz'), 'fizzbuzz class present');
	t.equal(+el.html(), 12, 'YESSSS');

	t.end();
});

I don't think you need the trailing () in require('angular-node')(), this was written a long time ago, so I'm guessing things have changed since then.