fdo#84061 Fix setting text style sheet listeners in SdrTextObj

The code in SdrTextObj::ImpSetTextStyleSheetListeners is obviously not
working correctly.

The families of the stylesheets are appended to the name of the family
for further usage.
An encoded string looks like "STYLE_NAME|3     "
The family is then extracted by copying the first (length-6) bytes,
e.g., "STYLE_NAME" in this example. Then another copy starting a
position 1 is created, e.g., "TYLE_NAME". This string is cast to an
Int32. Since this is not possible, 0 is returned, and the originally
stored family is lost.

This patch corrects this behavior, and adds a unit test.

Change-Id: I60c0add6e4b670acbbc264cc77672452f282f737
Reviewed-on: https://gerrit.libreoffice.org/10818
Reviewed-by: Matúš Kukan <matus.kukan@collabora.com>
Tested-by: Matúš Kukan <matus.kukan@collabora.com>