Does this nodeunit test conform to best practices?
You could use sinon.js to have the power of spies, stubs and mocks. http://sinonjs.org/

That way you could create a stub of httpRequest.

Look for stub.yield([arg1, arg2, ...]).

Edit I added a code example. I did not fully tested the code, but it's how it would look like.

var sinon = require('sinon');
require('../server/extensions');
var FmsRescuers = require('../server/fms_rescuers');
var Rescuer = require('./stubs/helper').Rescuer;

var mockedHttpRequest;
var rescuers;
var rescuer;

exports.testCheckIn = {    
    setUp: function(done) {
        var httpRequest { post: function () {} };
        mockedHttpRequest = sinon.mock(httpRequest);            
        rescuers = new FmsRescuers(mockedHttpRequest);   
        rescuer = new Rescuer(42, 'Name', new Date().addHours(-2).toTimeZonedString(), new Date().addHours(-1).toTimeZonedString());

    },

    'check in requests correct path' : function(test) {
        var expectedPath = '/attendance/checkin'
        mockedHttpRequest
                .expects('post')
                .once()
                .withArgs(expectedPath)
                .yields('', 200);

        rescuers.checkIn(rescuer, 123, 3, function(result, status) {
            mockedHttpRequest.verify();
            test.equal(result, '');
            test.equal(status, 200);
            test.done();
        });
    },
};