Changes

testenv: fix 'container crashed' detection

* When the testsuite stopped and using podman, check if it stopped
  because the container crashed and raise an exception.

* Even after 9eb5e696 ("testenv: make podman stop more robust") it
  happens sometimes in jenkins that the container gets stopped on
  purpose because the testsuite is done, but then the watchdog process
  prints an error saying it crashed (without actually stopping testenv
  at this point). Change the message to a debug message that just says
  it stopped, this should not be an error.

Related: OS#6607
Change-Id: I3c116719cdc00a99e27e5237ba3d57d5258b0919