regression from export bugfix tdf#90190.
rLineProperties.maLineFill was set, but that value was
never used, leaving aBorderLine with the default value.

Setting borderline to COL_AUTO lets it merge in with background
colors while still taking up the space defined by the border width.

The existing unit test says "it is likely you will break
this if fixing background issues, so feel free to adjust the
test values." Indeed, this patch removes an unwanted black border,
so utilizing that test as a proof.

Change-Id: I0725a3d5874a509a4464c586d60c77cde2b49fcb
Reviewed-on: https://gerrit.libreoffice.org/55205Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Justin Luth <justin_luth@sil.org>