[#283] pkg/session: Add container context generator

Implement `sessiontest.ContainerContext` function for testing.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/session/test/container.go

@@ -0,0 +1,26 @@
+package sessiontest
+
+import (
+	"math/rand"
+
+	cidtest "github.com/nspcc-dev/neofs-api-go/pkg/container/id/test"
+	"github.com/nspcc-dev/neofs-api-go/pkg/session"
+)
+
+// ContainerContext returns session.ContainerContext
+// which applies to random operation on a random container.
+func ContainerContext() *session.ContainerContext {
+	c := session.NewContainerContext()
+
+	setters := []func(){
+		c.ForPut,
+		c.ForDelete,
+		c.ForSetEACL,
+	}
+
+	setters[rand.Uint32()%uint32(len(setters))]()
+
+	c.ApplyTo(cidtest.Generate())
+
+	return c
+}