pdl: Expand the list of tests for the python generator

- Expand the test grammar with struct tests, inheritance tests
- Extract the list of test vectors to a separate reference json file
  at test/canonical/tests.json
- Fix some generator bugs on the way :)

Test: atest --host --no-bazel pdl_python_generator_test
Change-Id: Idf6d82421d8d858198084dc589b2184ca523fda2