i18n: Add case mapping and case-insensitive string comparison tests.

i18n: Add case mapping and case-insensitive string comparison tests.

Thanks to Mark H Weaver <mhw@netris.org> for coming up with most of the
examples.

* test-suite/tests/i18n.test (%german-utf8-locale-name,
  %greek-utf8-locale-name): New variables.
  (under-german-utf8-locale-or-unresolved,
  under-greek-utf8-locale-or-unresolved): New procedures.
  ("text collation (German)", "text collation (Greek)"): New tests
  prefixes.
  ("string mapping")["string-locale-upcase German",
  "string-locale-upcase Greek", "string-locale-upcase Greek (two
  sigmas)", "string-locale-downcase Greek", "string-locale-downcase
  Greek (two sigmas)"]: New tests.