test: check the next elapse timer timestamp after deserialization

When deserializing a serialized timer unit with RandomizedDelaySec= set,
systemd should use the last inactive exit timestamp instead of current
realtime to calculate the new next elapse, so the timer unit actually
runs in the given calendar window.

Provides coverage for:
  - https://github.com/systemd/systemd/issues/18678
  - https://github.com/systemd/systemd/pull/27752

test/units/util.sh

@@ -57,6 +57,15 @@ assert_le() {(
     fi
 )}
 
+assert_ge() {(
+    set +ex
+
+    if [[ "${1:?}" -lt "${2:?}" ]]; then
+        echo "FAIL: '$1' < '$2'" >&2
+        exit 1
+    fi
+)}
+
 assert_in() {(
     set +ex