Recognise ß properly as a lower-case letter (bug#11309)

Recognise ß properly as a lower-case letter (bug#11309)

ß was incorrectly treated as a caseless character and thus not matched
by the regexp [[:lower:]] (or, in case-folding mode, [[:upper:]]).
The reason is that the upcase table maps it to itself, which can be
remedied by mapping it to ẞ (U+7838) instead.  Doing so does not
affect upcasing since the special-uppercase property maps it to SS.

* lisp/international/characters.el (tbl): Map ß to ẞ in the upcase
table.
* test/src/regex-emacs-tests.el (regexp-eszett): Uncomment previously
failing tests.  Add checks to make sure that case transformations
remain valid.