REGTESTS: sample_fetches: test the TCP_INFO counters

REGTESTS: sample_fetches: test the TCP_INFO counters

fc_unacked, fc_sacked, fc_lost, fc_retrans, fc_fackets and
fc_reordering had no coverage. Their values depend on what happened on
the network, so, as the notes said, only their presence and their type
can be asserted, which is what the existing rtt test does for fc_rtt.

They get a file of their own rather than joining that test because
three of them do not exist on macOS, which forces an "#EXCLUDE_TARGETS"
marker that fc_rtt and fc_rttvar do not need.