When a tag is made known to the Validator, which it already knows, the
old id is returned and nothing is added.
As this has to be checked by iterating over all known tags, a new helper
function is written for both SH_Validator_check_tag and
SH_Validator_register_tag: SH_Validator_get_tag.

Because we want also to test this function, we have to include
validator.c in the test file and override the static keyword by an empty
macro. It isn't possible any more to check for correct overflowdetection
by setting the index to UINT_MAX, because it will be iterated over all
data, thus raising an SIGSEGV when doing so. This is solved by filling
garbage unil UINT_MAX is really reached. As there would be an timeout
and it would fill RAM with around 40 GB of garbage, UINT_MAX is
overriden prior to inclusion of validator.c .