Restructure builtin tests

- rename all test files to test_ - I find this very useful when I want
  to open the corresponding test file using my editors fuzzy file search
- place all tests in quite small top level function, which makes it easy
  to look at generate C code
- avoid exceptions for control flow, instead using simple if-checks and
  return - simpler code and we don't rely on working exceptions for most
  code... not that we'd expect exceptions not to work, but it's just
  unnecessary
- overall consolidation of tests and sort into where it makes sense
- more tests for bytes in particular

This is by no means perfect, but it's a start.