Set cert verify callback in mfl unit tests
Set cert verify callback in mfl unit tests

Previously the handshakes were actually failing because client mode
defaults to reject all certificates.
Also fixed a bug in the negotiate/shutdown helper functions in
s2n_client_extensions_test. The functions were not returning failures
correctly.

closes #589

gibson-compsci/s2n,PKRoma/s2n,alexeblee/s2n,raycoll/s2n,bpdavidson/s2n,colmmacc/s2n,jldodds/s2n,alexeblee/s2n,bpdavidson/s2n,PKRoma/s2n,alexeblee/s2n,gibson-compsci/s2n,colmmacc/s2n,raycoll/s2n,awslabs/s2n,wcs1only/s2n,colmmacc/s2n,gibson-compsci/s2n,gibson-compsci/s2n,bpdavidson/s2n,PKRoma/s2n,PKRoma/s2n,awslabs/s2n,wcs1only/s2n,wcs1only/s2n,awslabs/s2n,wcs1only/s2n,raycoll/s2n,PKRoma/s2n,wcs1only/s2n,alexeblee/s2n,jldodds/s2n,gibson-compsci/s2n,alexeblee/s2n,PKRoma/s2n,awslabs/s2n,wcs1only/s2n,wcs1only/s2n,bpdavidson/s2n,raycoll/s2n,alexeblee/s2n,colmmacc/s2n,bpdavidson/s2n,awslabs/s2n,PKRoma/s2n,raycoll/s2n,bpdavidson/s2n,colmmacc/s2n,raycoll/s2n,colmmacc/s2n,awslabs/s2n,gibson-compsci/s2n,wcs1only/s2n,jldodds/s2n,PKRoma/s2n,jldodds/s2n,jldodds/s2n