Changes

testenv: respawn.sh: don't respawn on segfault

Do not respawn the process if it segfaulted or aborted abnormally in
another way (exit code >= 128).

Change-Id: I6f3f6218095643609a8b8869b57dde6b6bdb77a0

testenv: print backtrace on startup fail too

When a program fails to start up, look for the coredump and print a
backtrace if it was found (instead of only doing it if a program
crashes later on).

Change-Id: I66a55dcfdab17d035ce6425220763a85b5b6dc90

testenv: build virtphy from src with --binary-repo

Add logic to build virtphy from source when running with --binary-repo.
This extra code path is needed because we currently don't have virtphy
packaged (like trxcon and sccp_demo_user), and we need to build the
libosmocore binary package instead of building completely from source as
we would do it with osmo-dev.

Use ".split(" ", 1)[0]" on the program= value to only look at its first
word, so we can later on use it in testenv.cfg file as follows:

  [virtphy]
  program=virtphy -s /tmp/osmocom_l2

Change-Id: I37bac8509b2601286e4feab099782f82c8338dca

bts: add initial testenv.cfg

As usually, imported from docker-playground and adjusted to run on the
loopback device.

Related: OS#6638
Change-Id: Id9ed3780d166c5f6736a006f466dc5f0dded6279