tko/job_serializer: Improve install dependencies notes

Make a generic mention of the packages needed to run
the unittest appropriately, rather than mentioning
specifics. Also, point out the install dependencies
script, that will install the right packages for
Ubuntu and Fedora at this point.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
tko/job_serializer_unittest.py: Print better error message

Print a more helpful message when the system lacks
the dependencies needed to run this unittest.

CC: Nishanth Aravamudan <nacc@linux.vnet.ibm.com>
Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>