x11-toolkits/plasma5-kdeplasma-addons: add missing testlib dependency

• testlib got pulled in transitively, when WEBENGINE/PURPOSE is enabled. When both options are of, the required dependency is missing on the system and threfore leading to build failures.

PR: 264199
(cherry picked from commit 1854a7934a4f285dbc48b449e37cca784955b40b)