Changes

testenv: run daemons with 'ulimit -c unlimited'

Prepare to support getting core files without having systemd-coredump
installed, as we plan to uninstall it from the jenkins servers to make
retrieving coredumps for other jobs feasible again.

When starting daemons, set the maximum core file size to unlimited.
Otherwise it might be at 0, resulting in no core files getting
generated.

I have considered using resource.setrlimit() on the python process
instead, but this wouldn't work when spawning the daemons inside the
podman container.

Change-Id: Ideaf0386c8d6111c2634f276f926e976023ff511

testenv: coredump: support core_pattern=core

Support getting core files from a typical core_pattern=core where the
coredump just gets stored in the current working dir, instead of always
retrieving it from coredumpctl. This is what we will use with jenkins in
the future, as it makes getting core files in other jobs easier. Remove
support for the custom testenv-coredump-helper code that isn't needed
anymore.

Change-Id: Ia765b01432e4cb4cd36c45de874b966e3ebf55bc

testenv: add empty lines around backtrace

Make the backtrace stand out visually from the rest of the logs.

Change-Id: Icd75379a875b72ddf3364895bc6a0afc57d2901e