s2nd: Use a certificate chain by default
s2nd: Use a certificate chain by default

Previously was just a dummy leaf. Chain is needed for handshake
fragmentation integration test. Should be a no-op.

- Key size: RSA 2048
- Leaf CN: s2nTestServer
- Intermediate CN: s2nTestIntermediate
- Root CN: s2nTestRoot

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