Add Redis identity test

Run identity tests against a redis DbContext.

@lajones Can you review the redis stuff.  I copied this from Redis.Functional pretty much as is?  I had to disable 2 tests due to key generation not working.
We've had problems with multithreading where the tests share a context like this - so we're not recommending this pattern any more. DbContext is not thread-safe. If you have a look at e.g. the Redis FunctionalTests SimpleTest.cs file we now recreate the context for each test. E.g.:

using (var context = CreateContext(...))
{
    // rest of test
}