run: Process whether we can run tcpdump

When running as non root, or when tcpdump is not available,
we can't run it, so don't even bother trying.

This is implemented by a check in the test runner that
verifies whether:

1) tcpdump is available somewhere (we have a pretty paranoid
   search function)
2) We can run it (ie, we're root)

If not, just set run_tcpdump to 'no'. This will appear on
the summary of the cartesian parser options and in logs,
so I believe there's not much chance it can go unnoticed.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>