fix: remove frames from assert interface stack

- Make it so that only `AssertionError` is thrown from inside of an
  assert interface wrapper function

- Always set `ssfi` to the current function when creating a new
  `AssertionError` inside of an assert interface wrapper function

- Always set `ssfi` to the current function when creating a new
  `Assertion` inside of an assert interface wrapper function,
  and set the `keep_ssfi` flag

- Improve the `globalErr` test helper function to also validate that the
  thrown error's stack trace doesn't contain frames from the assert
  interface