Add a test of Field::to/form_file_format().
tests_field_file_format: Set the locale and test several locales.

* Makefile_tests.am:
* tests/test_field_file_format_in_locales.sh:
  Add a script that runs test_field_file_format in various locales,
  because GdaNumeric seems to be doing some locale-sensitive
  atof().
* tests/test_field_file_format.cc: Use the current locale.
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