Clarify src vs dest path mismatch in :symbolic_link_{absolute,relative}_body

Unfortunately kyua does not omit the path mismatch on failure, so it must be coded
into the error message.

Cache the values, run the test(1) call, then print out the values in an atf_fail
call to emit the required diagnostics to debug why things are failing.

MFC after: 1 week
Sponsored by: Dell EMC Isilon