Changes

testenv: make podman stop more robust

Terminate the watchdog process before killing the podman container. This
avoids bogus errors from a race condition where the container gets
killed first, and then the watchdog process tries to feed the watchdog
and fails:

  [testenv] Stopping podman container
  [testenv] + ['podman', 'kill', 'testenv-hnbgw-all-osmocom-latest-20241031-1222-f34534a5-1']
  e41700779a8ca5daf18ac5daa27d59a84d8442196e352f2756a19baf0592cf89
  Error: no container with name or ID testenv-hnbgw-all-osmocom-latest-20241031-1222-f34534a5-1 found: no such container
  [testenv] podman container crashed!

While at it, use "check=False" with the "podman kill" command, so we
avoid additional error messages if the container was already killed at
that point (could happen through a bug). If we fail to kill it here, it
is not a problem because the watchdog will ensure it terminates shortly
after the watchdog process was stopped.

Related: OS#6607
Change-Id: I6c67273af5b6a80ae0c94e14cb92e3784807c21d