Remove use of getClass in secondary constructors (#28355)

* Remove use of getClass in secondary constructors

As this is not allowed anymore on newer versions of scala, and likely
didn't work correctly in the past either

This might make selecting unique actor system names for test
actor systems a bit less reliable, but that didn't seem to be
critical anyway.

Thanks to @som-snytt for the heads-up and initial implementation
in #28353

* Avoid AkkaSpec.getCallerName in MultiNodeClusterShardingConfig

* StreamSpec can be abstract

* Use more sophisticated test class name logic

* scalafmt

akka-cluster-sharding/src/multi-jvm/scala/akka/cluster/sharding/MultiNodeClusterShardingSpec.scala

@@ -45,6 +45,10 @@ object MultiNodeClusterShardingSpec {
 
 }
 
+/**
+ * Note that this class is not used anywhere yet, but could be a good starting point
+ * for new or refactored multi-node sharding specs
+ */
 abstract class MultiNodeClusterShardingSpec(val config: MultiNodeClusterShardingConfig)
     extends MultiNodeSpec(config)
     with MultiNodeClusterSpec {