How do I simulate an ADFS IdP to facilitate the testing of my SP code

Azure Active Directory should be very similar to implementations in ADFS (and the federation part is likely identical) and should be just fine for testing of your implementation. You can setup a free trial account for Microsoft Azure which includes the Azure Active Directory.

Information about SAML endpoints and SSO process can be found in the Azure documentation.

Other option is test against another SAML compatible Identity Provider, have a look at SSO Circle.

Microsoft also offers a Microsoft Partner Network Action Pack which enables companies to access a range of software (including Windows Server with ADFS) with development licenses for a reasonable price (few hundred euros). It might be your other option.