Add a function for parsing RFC 5280's "TBSCertificate".

net/cert/internal/test_helpers.cc

Index: net/cert/internal/test_helpers.cc
diff --git a/net/cert/internal/test_helpers.cc b/net/cert/internal/test_helpers.cc
index 91feb80604f505b3f98d9d929317272d6eeb0ea3..0b5363d8acc7da720b37c2698226f91a189a9c0a 100644
--- a/net/cert/internal/test_helpers.cc
+++ b/net/cert/internal/test_helpers.cc
@@ -83,7 +83,7 @@ der::Input InputFromString(const std::string* s) {
 
   // Ensure that all specified blocks were found.
   for (const auto& mapping : mappings_copy) {
-    if (mapping.value) {
+    if (mapping.value && !mapping.optional) {
       return ::testing::AssertionFailure() << "PEM block missing: "
                                            << mapping.block_name;
     }