Test CRYPTO_gcm128_tag in gcm_test.cc.

CRYPTO_gcm128_encrypt should be paired with CRYPTO_gcm128_tag, not
CRYPTO_gcm128_finish.

Change-Id: Ia3023a196fe5b613e9309b5bac19ea849dbc33b7
Reviewed-on: https://boringssl-review.googlesource.com/c/34265
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@google.com>

crypto/fipsmodule/modes/gcm_test.cc

diff --git a/crypto/fipsmodule/modes/gcm_test.cc b/crypto/fipsmodule/modes/gcm_test.cc
index a6b7fc0..16d9d09 100644
--- a/crypto/fipsmodule/modes/gcm_test.cc
+++ b/crypto/fipsmodule/modes/gcm_test.cc
@@ -90,7 +90,10 @@
       CRYPTO_gcm128_encrypt(&ctx, &aes_key, plaintext.data(), out.data(),
                             plaintext.size());
     }
-    ASSERT_TRUE(CRYPTO_gcm128_finish(&ctx, tag.data(), tag.size()));
+
+    std::vector<uint8_t> got_tag(tag.size());
+    CRYPTO_gcm128_tag(&ctx, got_tag.data(), got_tag.size());
+    EXPECT_EQ(Bytes(tag), Bytes(got_tag));
     EXPECT_EQ(Bytes(ciphertext), Bytes(out));
 
     CRYPTO_gcm128_setiv(&ctx, &aes_key, nonce.data(), nonce.size());