Use a more unique name for the cover variable

[ixdy]

In kubernetes, we get a build failure when running bazel coverage:

GoCompile: error running subcommand: exit status 2
pkg/kubelet/container/testing/os.go:143: Cover_os redeclared in this block
        previous declaration during import "k8s.io/kubernetes/pkg/kubelet/container"
pkg/kubelet/container/testing/runtime_mock.go:32: Cover_os redeclared during import "k8s.io/kubernetes/pkg/kubelet/container"
        previous declaration at pkg/kubelet/container/testing/os.go:143

The root cause appears to be at least partially self-inflicted; we have a pkg/kubelet/container/os.go file (in package container), and a pkg/kubelet/container/testing/os.go file (in package testing).

The existing rules_go coverage generator creates a Cover_os variable in each package, which would be unique, except that pkg/kubelet/container/testing/runtime_mock.go dot-imports pkg/kubelet/container, causing the conflict.

I've attempted to fix the issue here by just including the package path in the variable name, e.g. Cover_k8s_io_kubernetes_pkg_kubelet_container_os instead of Cover_os. This seems to work, though I'm not sure if there are corner cases I'm missing.

[jayconrod]

Thanks!