Fix decoder error on SM2 private key

Added sm2 testcases to endecode_test.c.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/25266)

(cherry picked from commit 25bd0c77)