linuxnamespaces.run_in_fork: use os._exit instead of sys.exit

When using sys.exit, we actually raise a SystemExit exception and as a
consequence exit all context managers. If a particular context manager
pertains only the process at hand, we don't really care, because our
process is supposed to vanish. If a context manager changes external
state such as tempfile.NamedTemporaryFile, this is very bad and
unexpected. We need to ensure that such cleanup is not performed.

This also simplifies the test suite that had to emulate this behaviour
already as pytest uses a context manager.

0 files changed, 0 insertions, 0 deletions