adds a simple test for the result class
that tests how multiple entries from the extractors
with the same property are merged into a list.
I was recently wondering if this is actually desired behavior. Right now it is definitely required since KFileMetaData wrongly outputs QStrings instead of a QStringList, but when this is fixed, this behavior should be removed imho. Otherwise, querying metadata via KFileMetaData and via Baloo differs in output, which it shouldn't.