Fix autosizing broken in 3c72396a36 and
not fully fixed by f7e335c031.
Simplify the code to make it more obviously correct, by separating the
computation of the extent suitable for the label and determining the
size to use taking into account the extents of both the column data and
the its column.
Also add the unit test checking that auto-sizing works correctly in all
the different cases.
Closes https://github.com/wxWidgets/wxWidgets/pull/1600
Co-Authored-By: Ilya Sinitsyn <the_siv@mail.ru>