add test for not displaying mapped symbols

If a symbol A is mapped to another symbol B, we should
not display (or actually register) symbol A for the
ontology to which the mapping is applied.
ensure non-exec of symbol callback in link

When in a symbol-mapping link (DGLink which contains
a map element) we should not execute the callback
for a symbol, as this will create the symbol
as part of the current ontology. And this not really
what we want.

Oh by the way: The spec is green.
Merge pull request #823 from ontohub/767-avoid_symbol_creation_caused_by_mapping

767 avoid symbol creation caused by mapping