org-table-sort-lines: Fix case-sensitive sorting

org-table-sort-lines: Fix case-sensitive sorting

* org-table.el (org-table-sort-lines): Fix case sensitive sorting,
improve docstring.
* test-org-table.el (test-org-table/sort-lines): Enforce C locale when
testing alphabetic sorting.

‘sort-subr’ ignores ‘sort-fold-case’ when a predicate is provided. To
correctly handle case-sensitivity, we now bake it into the predicate.

Since we are now sorting according to the user’s locale, WITH-CASE
will not make a difference in most instances, since most locales
always sort case-insensitively (cf. how GNU sort ignores the ‘-f’
switch).  We now mention this in the function docstring.

In order to meaningfully test case-sensitive sorting, we now enforce
the C locale in the respective unit test.