Add a basic test for BN_bn2dec.

This would have caught 099e2968ed3c7d256cda048995626664082b1b30. This is
a port of the test added in
https://boringssl.googlesource.com/boringssl/+/7c040756178e14a4d181b6d93abb3827c93189c4

Reviewed-by: Emilia Käsper <emilia@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1496)