[fuzzer] Adds code generation target. (#8795)

* adds code generation fuzzer target.

* add buffer verification

* add table verification in codegen fuzzer

---------

Co-authored-by: Björn Harrtell <bjornharrtell@users.noreply.github.com>

tests/fuzzer/flatbuffers_parser_fuzzer.cc

@@ -45,13 +45,20 @@ extern "C" int LLVMFuzzerTestOneInput(const uint8_t* data, size_t size) {
   auto parse_input = input.c_str();
 
   // Check Parser.
-  parser.Parse(parse_input);
-  // TODO:
-  // Need to add additional checks for inputs passed Parse(parse_input)
-  // successfully:
-  // 1. Serialization to bfbs.
-  // 2. Generation of a default object.
-  // 3. Verification of the object using reflection.
-  // 3. Printing to json.
+  auto result = parser.Parse(parse_input);
+  if (result) {
+    parser.Serialize();
+    const uint8_t* buf = parser.builder_.GetBufferPointer();
+    flatbuffers::Verifier verifier(buf, parser.builder_.GetSize());
+    TEST_EQ(true, reflection::VerifySchemaBuffer(verifier));
+
+    auto root = flatbuffers::GetRoot<flatbuffers::Table>(buf);
+    if (verifier.VerifyTableStart(buf) && root->VerifyTableStart(verifier)) {
+      if (parser.root_struct_def_) {
+        std::string json_output;
+        flatbuffers::GenText(parser, buf, &json_output);
+      }
+    }
+  }
   return 0;
 }