tests: rename stream name in tests

tests: rename stream name in tests

PacketGenerator streams were named based on their sw_if_index but without
knowing that, it looked like it was paired with the interface instance value.

Reencode the name to encode the data in a clearer way.

20:48:12,859 CLI: packet-generator new pcap
  /tmp/vpp-unittest-TestClassifierPBR-6DVQW2/pg0_in.pcap
  source pg0 name pcap0-sw_if_index-1

Change-Id: I5b82193943fee0cf060900d73aaaaecca1247105
Type: style
Signed-off-by: Paul Vinciguerra <pvinci@vinciconsulting.com>