Changes

testenv-coredump-helper: remove

After looking at this again, it makes more sense to keep
/proc/sys/kernel/core_pattern as "core" on build4 and build5, which we
use for jenkins jobs instead of installing systemd's coredumpctl and
making all coredumps go through that. The motivation for using
coredumpctl in testenv was that lots of developers may have it already
installed (though I'm not sure about that anymore), and that it gives a
nice API for retrieving the related executable name to a coredump
instead of having to parse the output of e.g. "file".

Using coredumpctl had the disadvantage that coredumps for all other
jobs, such as the master-builds are also caught by that and cannot be
easily placed in the workspace directory. I have started implementing
this by extending testenv-coredump-helper to communicate through a
socket and mounting that socket inside docker containers running
contrib/jenkins.sh (host -> lxc -> docker), and it works, but this
complexity is not useful here.

Instead the related patch will make testenv pick the core files when
coredumpctl is not available, and read the path from "file", making
testenv-coredump-helper obsolete.

Related: osmo-ttcn3-hacks Ia765b01432e4cb4cd36c45de874b966e3ebf55bc
Change-Id: I2959c6e6d97d5691ee2a4ae5d49a351eb5811f10

scripts/wrapper_core_bt_on_error: new script

Add a wrapper script that runs a given program, and checks the exit
code. If it is not 0, and a core file appears, then load the coredump in
gdb and display its backtrace.

Change-Id: I9673abf3ae3b154505ea09237d37d7da4bf5d57f

jobs/master,gerrit: use wrapper_core_bt_on_error

When the exit code isn't 0, find coredumps and display their backtraces.

Change-Id: Idc893969b158e11d5ae17d0c0a05b52e662f8929

jobs/master,gerrit: set CFLAGS=-g

Generate debug symbols, so backtraces from coredumps become more useful.

Change-Id: Ieff8670e376f140a672a800961be753028cf7b6f