test WICS.pm

Test the methods of WICS.pm for correctness. This entails duplication
of code from the test files for each converter, but I believe it is
still important, since I have previously found a bug in this file
that would have been prevented via tests.
return documents from converters, not strings

Instead of returning string pointers from each of the conversion
routines, return the ITS::DOM object, which can then be stringified
via the string method. This will allow post-processing without
re-parsing the HTML output.
rename variable to better reflect contents

The results of the conversion routines were stored in "$html" variables
in WICS and WICS-GUI. However, the conversion results are not always
HTML (there is also XLIFF output, and there might be others in the
future). Rename both instances of "$html" to "$results" to better
reflect the variables' contents.