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

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

Fixes: f995e5bd ("TEST: Add provider_fallback_test, to test aspects of
fallback providers")
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/25108)

(cherry picked from commit 6e8a1031)