This integration test allows us to detect if a given feature gate will
panic kubeadm. This builds on the assumption that a golang panic makes
the process exit with the code 2.
These tests are not trying to check if the init process succeeds or
not, their only purpose is to ensure that the exit code of the
`kubeadm init` invocation is not 2, thus, reflecting a golang panic.
Some refactors had to be made to the test code, so we return the exit
code along with stdout and stderr.
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