Add an EVP_MD_CTX_md() test

Reviewed-by: Richard Levitte <levitte@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8614)

test/evp_extra_test.c

@@ -1083,7 +1083,8 @@ static int calculate_digest(const EVP_MD *md, const char *msg, size_t len,
             || !TEST_true(EVP_DigestUpdate(ctx, msg, len))
             || !TEST_true(EVP_DigestFinal_ex(ctx, out, NULL))
             || !TEST_mem_eq(out, SHA256_DIGEST_LENGTH, exptd,
-                            SHA256_DIGEST_LENGTH))
+                            SHA256_DIGEST_LENGTH)
+            || !TEST_true(md == EVP_MD_CTX_md(ctx)))
         goto err;
 
     ret = 1;