pruning: fix tests + implement 'not_replicated' and 'keep_regex' keep rule

tests expected that a KeepRule returns a *keep* list whereas it
actually returns a *destroy* list.

pruning/keep_not_replicated.go

@@ -0,0 +1,15 @@
+package pruning
+
+type KeepNotReplicated struct {
+	forceConstructor struct{}
+}
+
+func (*KeepNotReplicated) KeepRule(snaps []Snapshot) (destroyList []Snapshot) {
+	return filterSnapList(snaps, func(snapshot Snapshot) bool {
+		return snapshot.Replicated()
+	})
+}
+
+func NewKeepNotReplicated() *KeepNotReplicated {
+	return &KeepNotReplicated{}
+}