Unit test for https://gerrit.libreoffice.org/21021/

Add unit test for Engineering format
Add unit test for optional sign in exponent

Change-Id: I958ccdcc68a4e496cbd4bdc74d0650c133f3a3f7
Reviewed-on: https://gerrit.libreoffice.org/21824Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Markus Mohrhard <markus.mohrhard@googlemail.com>