test: Fix SELinux context of fake /proc/cmdline
test: Fix SELinux context of fake /proc/cmdline

This avoids violations of systemd-gpt-generator and possibly other
things that try to parse it.

Closes #14070
cockpit-project/cockpit,deryni/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,deryni/cockpit,mvollmer/cockpit,deryni/cockpit,moraleslazaro/cockpit,moraleslazaro/cockpit,garrett/cockpit,moraleslazaro/cockpit,mvollmer/cockpit,martinpitt/cockpit,garrett/cockpit,garrett/cockpit,martinpitt/cockpit,mvollmer/cockpit,cockpit-project/cockpit,mvollmer/cockpit,martinpitt/cockpit,mvollmer/cockpit,martinpitt/cockpit,moraleslazaro/cockpit,cockpit-project/cockpit,moraleslazaro/cockpit,martinpitt/cockpit,deryni/cockpit,deryni/cockpit,garrett/cockpit,cockpit-project/cockpit,cockpit-project/cockpit,deryni/cockpit,deryni/cockpit,garrett/cockpit