Report Tests: Use a specific locale, so these work everywhere.

* glom/libglom/data_structure/glomconversions.[h|cc]:
  Add a get_text_for_gda_value() overload that takes a locale name,
  so we do not need to just use the current locale.
* glom/report_builder.cc: Use the new overload instead of ignoring
  the provided locale for this.
* tests/test_selfhosting_new_then_report.cc
* tests/test_selfhosting_new_then_report_summary.cc:
  Pass the en_US.UTF-8 locale name to the ReportBuilder constructor,
  instead of depending on the system's current locale,
  so this tests can pass regardless of the current locale.
DbUtils: get_database_preferences(): Use a specific locale.