Add ability to the SAML test provider to create signed SAML responses

[hcjulz]

This PR adds the ability to the SAML test provider to create signed SAML responses. It uses the goxmldsig library for this purpose, which is also used by the underlaying gosaml2 library used in cap/saml.

Before, this library used samltest.id (a test SAML Identity provider), which doesn't exist anymore. The tests for parsing SAML responses have been updated to use the test provider.

I'll open a follow up PR to improve the test SAML identity provider in cap/saml. Areas to improve:

• Provide more options to modify the SAML response, such that we can get rid of any hard coded SAML response XML strings.
• Create the IDP Metadata programatically with options.

[jimlambrt]

ty!