test: write file from systemd service in transient unit

test: write file from systemd service in transient unit

This integration test demonstrates that a containerized systemd instance can
write to a bind mounted file observable to the host. Specifically, the bash
script uses systemd-run to start a systemd instance as a transient unit
container. This systemd-run command bind mounts a directory the container will
share with the host, and runs an internal service which creates and writes to a
file from the container's view of this directory. When finished writing, the
service runs the exit target, terminating the internal systemd instance, and
ending the lifetime of the container.

The script waits for the container to finish running, then verifies that the
expected file contents were written on the host side of the filesystem mount.

This test employs a workaround, creating an unmasked procfs mount on the host
which enables the privileged guest to create its own mounts internally. This
may indicate a systemd bug, as the privileged container should not rely on
the existence of an unmasked procfs on the host in order to mount its own
filesystems internally.