Changes

ttcn3-tcpdump-start: fix default TTCN3_PCAP_PATH

Fix that manually running testsuites (as described in README.md) would
unsucessfully attempt to start dumpcap for 10s before running each test:

  NOTE: unable to use dumpcap due to missing permissions in /tmp
  Warning: Named pipe already exists: /tmp/cmderr
  sudo -n /usr/bin/tcpdump -U -s 1520 -n -i any -w "/tmp/smdpp_Tests.TC_rsp_complete_flow.pcap" >/tmp/smdpp_Tests.TC_rsp_complete_flow.pcap.stdout 2>/tmp/cmderr &
  Waiting for packet dumper to start... 0
  Waiting for packet dumper to start... 1
  Waiting for packet dumper to start... 2
  Waiting for packet dumper to start... 3
  Waiting for packet dumper to start... 4
  Waiting for packet dumper to start... 5
  Waiting for packet dumper to start... 6
  Waiting for packet dumper to start... 7
  Waiting for packet dumper to start... 8
  Waiting for packet dumper to start... 9
  Packet dumper didn't start filling pcap file after 10 seconds!!!

This happens because /tmp is owned by root and not by the user running
ttcn3-tcpdump-start. Put the pcaps in /tmp/pcap by default instead of
/tmp and create this directory as the current user if it does not exist.
This also prevents cluttering /tmp.

Note that this is only relevant when manually running the testsuites and
not setting TTCN3_PCAP_PATH, e.g. testenv sets this variable already.

Change-Id: I1eb80d7f9e8dd491387e4d1da9b4ac6ac5cb7dc5