AbstractInMemoryDataFixture.cs

using NHibernate;
using NUnit.Framework;

namespace UnitTests
{
    public abstract class AbstractInMemoryDataFixture
    {
        private ISession session;

        [SetUp]
        public void BaseSetup()
        {
            session = InMemorySessionFactoryProvider.Instance.OpenSession();
        }

        [TearDown]
        public void BaseTearDown()
        {
            if (session != null)
                session.Dispose();
        }

        protected ISession Session
        {
            get { return session; }
        }
    }
}

InMemorySessionFactoryProvider.cs

using FluentNHibernate.Cfg;
using FluentNHibernate.Cfg.Db;
using NHibernate;
using NHibernate.Cfg;
using NHibernate.Tool.hbm2ddl;
using MyWebProject;

namespace UnitTests
{
    public class InMemorySessionFactoryProvider
    {
        private static InMemorySessionFactoryProvider instance;
        public static InMemorySessionFactoryProvider Instance
        {
            get { return instance ?? (instance = new InMemorySessionFactoryProvider()); }
        }

        private ISessionFactory sessionFactory;
        private Configuration configuration;

        private InMemorySessionFactoryProvider() { }

        public void Initialize()
        {
            sessionFactory = CreateSessionFactory();
        }

        private ISessionFactory CreateSessionFactory()
        {
            return Fluently.Configure()
                    .Database(SQLiteConfiguration.Standard.InMemory().ShowSql())
                    .Mappings(m => m.FluentMappings.AddFromAssemblyOf<TypeInMyWebProject>())
                    .ExposeConfiguration(cfg => configuration = cfg)
                    .BuildSessionFactory();
        }

        public ISession OpenSession()
        {
            ISession session = sessionFactory.OpenSession();

            var export = new SchemaExport(configuration);
            export.Execute(true, true, false, session.Connection, null);

            return session;
        }

        public void Dispose()
        {
            if(sessionFactory != null)
                sessionFactory.Dispose();

            sessionFactory = null;
            configuration = null;
        }
    }
}

TestSetupFixture.cs

using NUnit.Framework;

namespace UnitTests
{
    [SetUpFixture]
    public class TestSetupFixture
    {
        [SetUp]
        public void Setup()
        {
            InMemorySessionFactoryProvider.Instance.Initialize();
        }

        [TearDown]
        public void TestTeardown()
        {
            InMemorySessionFactoryProvider.Instance.Dispose();
        }
    }
}