dst: close most recent ColumnStore instead of first on defer

We were closing the incorrect column store at the end of the test (in some
cases double closing).

dst/dst_test.go

@@ -338,7 +338,6 @@ func TestDST(t *testing.T) {
 		}, walTicker,
 	)
 	require.NoError(t, err)
-	defer c.Close()
 
 	ctx := context.Background()
 	var db atomic.Pointer[frostdb.DB]
@@ -482,6 +481,10 @@ func TestDST(t *testing.T) {
 	t.Log("snapshot files:", listFiles("snapshots"))
 	t.Log("WAL files:", listFiles("wal"))
 
+	// Defer a close here. This is not done at the start of the test because
+	// the test run itself may close the store.
+	defer c.Close()
+
 	timestampSum := &int64checksum{}
 	readTimestamps := make(map[int64]int)
 	expectedTimestamps := make(map[int64]struct{})