Ensure journald events tests only run where supported

We don't support the journald events backend on RHEL8, for
example. So we can't unconditionally run these tests.

Partial fix for RHBZ1955166

Signed-off-by: Matthew Heon <mheon@redhat.com>

test/system/090-events.bats

@@ -81,6 +81,7 @@ function _events_disjunctive_filters() {
 
 @test "events with disjunctive filters - journald" {
     skip_if_remote "remote does not support --events-backend"
+    skip_if_journald_unavailable "system does not support journald events"
     _events_disjunctive_filters --events-backend=journald
 }