- as per pyxform/XLSForm#439, should always output a label node even if there
  is no label specified.
- pyxform had a warning for no label but this is removed since there is
  no requirement for a label.
- add tests to verify that a label is output and no warning raised
- fix existing tests that either expected a warning or no label

- some clients hide the guidance hint by default, in which case the user
  would see nothing / a small (i) symbol if a question had only a
  guidance hint without any label (or media) or hint
- update test_guidance_hint.py tests accordingly
- change newly added test from ss_structure to md-style
- removed e2e tests and associated files; more or less the same as the
  test immediately above in test_sheet_columns.py

- label check block applies to questions and other item types such as
  groups and repeats, so may be valid in other scenarios
- for example this block is referenced in XLSForm#542
- will require further changes to exclude question items from the check
  since that is the intent in XLSForm#439

- as noted in the comment, the check doesn't apply to questions anymore
  since PR XLSForm#543 makes pyxform always emit a label, and the only case
  that is not permissible is a guidance_hint with no label or hint.
- nesting this check inside the "control group" processing branch seems
  the only neat+easy way about this, since:
  - the context is a loop from which the various branches continue, so
    it can't check at the end,
  - each branch parses / determines the actual item type which is not
    apparent in the raw "type" data from the row dict, and
  - after the loop context exits, the data is in an array with nested
    dicts so it's no longer possible to determine the sheet row number
    for the error message to be as useful to users.
- refactored an old test which covers a lot of question types, into
  the tests_v1 / md style since this test is affected by the changes
  and having md style is more transparent
- added some tests to verify the assumption that for the label check,
  it's not going to encounter a None or empty value for "control_type".