move cmd/pruning to pruning, as it's independent of the command implementation

pruning/keep_helpers_test.go

@@ -0,0 +1,22 @@
+package pruning
+
+import (
+	"github.com/stretchr/testify/assert"
+	"testing"
+)
+
+func TestShallowCopySnapList(t *testing.T) {
+
+	l1 := []Snapshot{
+		stubSnap{name: "foo"},
+		stubSnap{name: "bar"},
+	}
+	l2 := shallowCopySnapList(l1)
+
+	assert.Equal(t, l1, l2)
+
+	l1[0] = stubSnap{name: "baz"}
+	assert.Equal(t, "baz", l1[0].Name())
+	assert.Equal(t, "foo", l2[0].Name())
+
+}