Extract some span_parser tests into "golden" tests (#4325)
* Extract some span_parser tests into more complete "golden" tests
The ".dart" files in test_data/syntax_highlighting are run through the syntax highlighting and dumped as ".dart.golden" files which annotate the results for each line.
* Remove invalid code that's failing bot
There's an existing test covering this left in `span_parser_test.dart`.
* Update goldens 🙈
* Add copyright to test_data + regen goldens
* Move syntax highlighting goldens into test/goldens
