Unittest the whitespace stripping logic for tox text generation

The separate class allows to unittest this functionality in isoloation.

Change-Id: I1e5eddfb455ca85a662ea38c03302883decc5d58
Reviewed-on: https://gerrit.libreoffice.org/9608
Tested-by: Caolán McNamara <caolanm@redhat.com>
Reviewed-by: Caolán McNamara <caolanm@redhat.com>