test.js

contract('Test', function(accounts) {
	it('should increment', function(done) {
		var test = Test.deployed();
		test.increment().then(function(ret) {
			return test.i.call();
		}).then(function(ret) {
			assert.equal(ret.valueOf(), 1);
			return test.increment();
		}).then(function(ret) {
			return test.i.call();
		}).then(function(ret) {
			assert.equal(ret.valueOf(), 2);
		}).then(done).catch(done);
	});
});

Test.sol

contract Test {
	uint public i;

	function increment() {
		i += 1;
	}
}