tdf#95356: unit test about text decorations for svg export

1) Fixed the trailing space issue in text decoration attribute value.

2) Changed the SVG export suite in order to make it more reusable for
other tests.

3) Added a unit test for testing that text decorations are exported
correctly.

Reviewed-on: https://gerrit.libreoffice.org/19870Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Andras Timar <andras.timar@collabora.com>
(cherry picked from commit 50506421)

Conflicts:
	sd/qa/unit/SVGExportTests.cxx

Change-Id: I2bd71974242a0007726fbdd5ef5637a9ec62fd47

filter/source/svg/svgwriter.cxx

@@ -811,13 +811,17 @@ void SVGTextWriter::addFontAttributes( bool bIsTextContainer )
             if( eCurFontUnderline != eParFontUnderline )
             {
                 if( eCurFontUnderline != UNDERLINE_NONE )
-                    sTextDecoration = "underline ";
+                    sTextDecoration = "underline";
                 bIsDecorationChanged = true;
             }
             if( eCurFontStrikeout != eParFontStrikeout )
             {
                 if( eCurFontStrikeout != STRIKEOUT_NONE )
-                    sTextDecoration += "line-through ";
+                {
+                    if( !sTextDecoration.isEmpty() )
+                        sTextDecoration += " ";
+                    sTextDecoration += "line-through";
+                }
                 bIsDecorationChanged = true;
             }