ecc: Fix regression in keygrip computation for cv25519.

* cipher/ecc-curves.c (domain_parms): Revert g_y for cv25519.
* tests/keygrip.c: Add test case for cv25519.

Note that the regression is only in master and not in 1.8.

• Regression-due-to: c3f39b6b55db618f77e983125072081c15bfbdd6
• GnuPG-bug-id: T4712
• Signed-off-by: Werner Koch <wk@gnupg.org>