Fix incorrect selection flags for ec serializer.

Fix incorrect selection flags for ec serializer.

Fixes #12630

ec_import requires domain parameters to be part of the selection.
The public and private serialisers were not selecting the correct flags so the import was failing.
Added a test that uses the base provider so that a export/import happens for serialization.

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/12681)