test/provider_test.c: Add OSSL_PROVIDER_unload() to avoid memory leak

Add OSSL_PROVIDER_unload() when OSSL_PROVIDER_add_builtin() fails to avoid memory leak.

Fixes: 5442611d ("Add a test for OSSL_LIB_CTX_new_child()")
Signed-off-by: Jiasheng Jiang <jiashengjiangcool@outlook.com>

Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/25109)

(cherry picked from commit 55662b67)