Fix systemd skip in unit tests

[tnozicka]

Description of your changes:
When we switched to UBI the part that skipped the broken systemd unit test stopped working with
daemon_test.go:39: Process org.freedesktop.systemd1 exited with status 1 which is not very descriptive.

dbus.Error{Name:"org.freedesktop.DBus.Error.Spawn.ChildExited", Body:[]interface {}{"Process org.freedesktop.systemd1 exited with status 1"}}

There is a bunch of things that are not there and we try to half use them so this PR simplifies the check similar to sd_booted check a looks whether /run/systemd/system/ directory exists.

This currently blocks the whole CI on unit tests.

Which issue is resolved by this Pull Request:
Resolves https://prow.scylla-operator.scylladb.com/view/gs/scylla-operator-prow/pr-logs/pull/scylladb_scylla-operator/2116/pull-scylla-operator-master-unit/1833408548160147456#1:build-log.txt%3A116

[zimnx]

/lgtm

[scylla-operator-bot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: tnozicka, zimnx

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[tnozicka]

#2118 (comment)
/retest