Update `RuntimePluginConfigSpec` plugin test to use `DurableState`

Once https://github.com/apache/pekko-persistence-r2dbc/pull/194 and https://github.com/apache/pekko/issues/1798 are done the `RuntimePluginConfigSpec` should be updated to use `DurableState` instead of testing its plugin directly here: https://github.com/apache/pekko-persistence-r2dbc/blob/16729ecddb0ef45303d9bdc6b0ba2230fc0417ff/core/src/test/scala/org/apache/pekko/persistence/r2dbc/journal/RuntimePluginConfigSpec.scala#L304-L319