The models:

class User < ActiveRecord::Base

  has_and_belongs_to_many :projects

end

class Project < ActiveRecord::Base

  has_and_belongs_to_many :users

end

The test:
----------
require File.dirname(__FILE__) + '/../test_helper'

class UserTest < Test::Unit::TestCase
  fixtures :projects
  fixtures :users
  fixtures :projects_users
  self.use_instantiated_fixtures  = true

  def test_create
    u = User.new
    u.login = 'fred'
    u.email = '[email protected]'
    assert u.save
  end

  def test_has_projects
    projects = @user_one.projects
    assert_equal 2, projects.length
  end

  def test_can_remove_projects
    projects = @user_one.projects
    assert_equal 2, projects.length
    @user_one.projects.delete(projects[0])
    u = User.find_by_login(@user_one.login)
    projs = u.projects
    assert_equal 1, projs.length
  end

end