tests: Cope with broken Libgcrypt versions

* common/t-sexputil.c (test_ecc_uncompress): Ignore unknwon curve
errors.

For unknown reasons some versions of Fedora or RHEL provide a patched
version of Libgcrypt with Brainpool support removed. We better ignore
this error because it is a regression in 2.2.28 although that older
versions could not used Brainpool keys.

• GnuPG-bug-id: T5502
• Signed-off-by: Werner Koch <wk@gnupg.org>