Add test certs and test for ECDSA cert and EC/RSA dual mode.

I'm also adding certs for IDNA 2003/2008 tests and simplify some test
data handling.
Signed-off-by: Christian Heimes <christian@python.org>

Issue #8109: The ssl module now has support for server-side SNI, thanks to a :meth:`SSLContext.set_servername_callback` method.
Patch by Daniel Black.