A gap in the kmc-analyze module displayMap support was that it was not
possible to extend an existing mapping file, but only to rewrite one.
This caused issues because the mappings could change between runs, if
keyboards changed, meaning fonts would not be backward compatible. This
change guarantees an existing mapping file will have exactly the same
character mappings, and only adds new mappings that are found.
Also adds unit tests and enables for kmc-analyze.
Fixes: #9767