Use curve25519-sha256 kex where possible. Except where we're explicitly

Use curve25519-sha256 kex where possible.   Except where we're explicitly
testing a different kex, use curve25519-sha256 since it's faster than the
default and supported even when configured without OpenSSL.  Add a check
to ensure that the kex we intended to test is the one we actually tested.
Speeds test up by ~5%.