crypto: ensure we use a predictable TLS priority setting

crypto: ensure we use a predictable TLS priority setting

The TLS test cert generation relies on a fixed set of algorithms that are
only usable under GNUTLS' default priority setting. When building QEMU
with a custom distro specific priority setting, this can cause the TLS
tests to fail. By forcing the tests to always use "NORMAL" priority we
can make them more robust.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>