=per #15671 fix local snapshot store picking up wrong file.

Added spec test to cover fix. To be reviewed by @ktoro
We usually run specs against both `leveldb` and `inmem` journals, as seen here: https://github.com/akka/akka/blob/master/akka-persistence/src/test/scala/akka/persistence/PersistentActorSpec.scala#L776 in case of akka-persistence specs. This can sometimes find races in tests etc. 

Not really required here I guess though so we could use `inmem,` which is cheaper. Maybe other hakkers would chime in about running on both journals?