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