advancedtls: Make test happy on Go1.15 (#4053)

diff --git a/security/advancedtls/advancedtls_test.go b/security/advancedtls/advancedtls_test.go
index a7ecf27..67c0b9d 100644
--- a/security/advancedtls/advancedtls_test.go
+++ b/security/advancedtls/advancedtls_test.go
@@ -867,8 +867,8 @@
 			if err != nil {
 				t.Fatalf("serverConfig.GetCertificate(clientHello) failed: %v", err)
 			}
-			if !cmp.Equal(*gotCertificate, test.wantCert, cmp.AllowUnexported(big.Int{})) {
-				t.Errorf("GetCertificates() = %v, want %v", *gotCertificate, test.wantCert)
+			if !gotCertificate.Leaf.Equal(test.wantCert.Leaf) {
+				t.Errorf("GetCertificates() returned leaf certificate does not match expected (-want +got):\n%s", cmp.Diff(test.wantCert, *gotCertificate, cmp.AllowUnexported(big.Int{})))
 			}
 		})
 	}