KeyMint: add separator for debugging output

When dumping attestation certificate chains with the --dump_attestations
option, add a separator between distinct chains to make them easier to
analyze.

Bug: 326564087
Test: VtsAidlKeyMintTargetTest
Change-Id: Ife19edfddef6c8cd26de9f9816c3c9bf65cbb929
diff --git a/security/keymint/aidl/vts/functional/KeyMintAidlTestBase.cpp b/security/keymint/aidl/vts/functional/KeyMintAidlTestBase.cpp
index 087f763..c121d31 100644
--- a/security/keymint/aidl/vts/functional/KeyMintAidlTestBase.cpp
+++ b/security/keymint/aidl/vts/functional/KeyMintAidlTestBase.cpp
@@ -2024,7 +2024,7 @@
         }
     }
 
-    if (KeyMintAidlTestBase::dump_Attestations) std::cout << cert_data.str();
+    if (KeyMintAidlTestBase::dump_Attestations) std::cout << "cert chain:\n" << cert_data.str();
     return AssertionSuccess();
 }