If scientific format is selected, "Thousands separator" option is almost
useless. It could be replaced by "Engineering Notation".

Rebase of https://gerrit.libreoffice.org/15152
Update with more robust tests.

Change-Id: Ie2b88b1f149fce26c32a43ace623cf1f45f38e6e
Reviewed-on: https://gerrit.libreoffice.org/15606Reviewed-by: Eike Rathke <erack@redhat.com>
Tested-by: Eike Rathke <erack@redhat.com>