Allow unknown fields in corpus parser

[nareddyt]

As discussed in envoyproxy/envoy#10796, this will allow breaking wire-compatibility changes in the input proto. The pre-existing corpus will still function, but old fields will be ignored.

Risk: Typos in the text proto will cause the fuzzer to run on an incomplete proto. Previously, this would log an error message and skip fuzzing with that test case.

Testing: I'm not sure how to add it to mutator_test.cc. I manually tested that unknown text format files will be parsed without ParseTextMessage returning false.

[nareddyt]

cc @asraa @vitalybuka

[nareddyt]

You may want to check this out and run the tests to verify my PR. I keep getting a test failure on my workstation:

96% tests passed, 1 tests failed out of 28

Total Test time (real) = 288.44 sec

The following tests FAILED:
         27 - test.libfuzzer_example_test (Failed)
Errors while running CTest
ninja: build stopped: subcommand failed.

Edit: Oh, it happens on master too. Nevermind

[nareddyt]

Newline matters here FYI (there shouldn't be one)

[vitalybuka]

Looks like we need to rollback this and fix issue in proto

Now it triggers stack overflows like

  | #5 0x816706 in google::protobuf::TextFormat::Parser::ParserImpl::SkipFieldValue() libprotobuf-mutator/build/external.protobuf/src/external.protobuf/src/google/protobuf/text_format.cc:827:9
  | #6 0x816d39 in google::protobuf::TextFormat::Parser::ParserImpl::SkipFieldValue() libprotobuf-mutator/build/external.protobuf/src/external.protobuf/src/google/protobuf/text_format.cc:830:11
  | #7 0x816d39 in google::protobuf::TextFormat::Parser::ParserImpl::SkipFieldValue() libprotobuf-mutator/build/external.protobuf/src/external.protobuf/src/google/protobuf/text_format.cc:830:11
  | #8 0x816d39 in google::protobuf::TextFormat::Parser::ParserImpl::SkipFieldValue() libprotobuf-mutator/build/external.protobuf/src/external.protobuf/src/google/protobuf/text_format.cc:830:11
  | #9 0x816d39 in google::protobuf::TextFormat::Parser::ParserImpl::SkipFieldValue() libprotobuf-mutator/build/external.protobuf/src/external.protobuf/src/google/protobuf/text_format.cc:830:11
  | #10 0x816d39 in google::protobuf::TextFormat::Parser::ParserImpl::SkipFieldValue() libprotobuf-mutator/build/external.protobuf/src/external.protobuf/src/google/protobuf/text_format.cc:830:11
  | #11 0x816d39 in google::protobuf::TextFormat::Parser::ParserImpl::SkipFieldValue() libprotobuf-mutator/build/external.protobuf/src/external.protobuf/src/google/protobuf/text_format.cc:830:11
  | #12 0x816d39 in google::protobuf::TextFormat::Parser::ParserImpl::SkipFieldValue() libprotobuf-mutator/build/external.protobuf/src/external.protobuf/src/google/protobuf/text_format.cc:830:11