comphelper: Use appropriate OUString functions on string constants

Change-Id: Id1d5c3cf2f76dbb33606cec1c0f17d4a1f282247
XmlTestTools: fix conversion from UTF-8 xmlChar strings to OUString

...which resolves the mystery of 0ba6360363fb73b5b200bbc486ed8eeac5f3d337
"Garbage in, garbage out?"

Change-Id: I51f102699d0474872c80392b27f71030b5e3fb59