Add test cases for epair

Implements the missing test cases for epair in a similar fashion to the
existing tests. Fixes shared abstractions to work with epair tests.

Submitted by: Ryan Moeller <ryan@freqlabs.com>
Reviewed by: asomers
MFC after: 2 weeks
Sponsored by: iXsystems, Inc.
Differential Revision: https://reviews.freebsd.org/D20498