Changes

ttcn3-bts-test: give bpftrace time to print stats

When running with bpftrace, send a kill signal to the bpftrace scripts
before killing the container. Otherwise the scripts don't print their
statistics, the end of the log then looks like this:

  [Thu Dec 18 18:06:06 UTC 2025] respawn: 2: starting: /data/bpftrace/udp_sendmsg_delay.sh
  + pidof osmo-bts-trx
  + bpftrace /data/bpftrace/udp_sendmsg_delay.bt -p 56
  Attaching 3 probes...
  Tracing udp_sendmsg() latency... Hit Ctrl-C to end.

Change-Id: I1781bddddf429c4c6a951b519a639b8663d397e6

ttcn3-bts-test: mount tracingfs only when needed

Change-Id: I59bbbc96c0686a2c8792626297e656df72e578ed