Add a regression test for curve25519.c r1.14

Add a regression test for curve25519.c r1.14

Generate random signatures of random messages and verify them. Then check
that the signature modified by adding the edwards25519 group order to the
upper half are rejected. This would not always be accepted without the
check in curve25519.c r1.14, but often enough that a few iterations suffice
to expose the missing check.