test: versioned constant getters (#2915)

## Context

During a 1:1 w/ @evan-forbes , we noticed that the versioned constant
getters don't have any unit tests.

pkg/appconsts/versioned_consts_test.go

@@ -0,0 +1,51 @@
+package appconsts_test
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"github.com/celestiaorg/celestia-app/pkg/appconsts"
+	"github.com/celestiaorg/celestia-app/pkg/appconsts/testground"
+	v1 "github.com/celestiaorg/celestia-app/pkg/appconsts/v1"
+	v2 "github.com/celestiaorg/celestia-app/pkg/appconsts/v2"
+)
+
+func TestSubtreeRootThreshold(t *testing.T) {
+	type testCase struct {
+		version uint64
+		want    int
+	}
+	testCases := []testCase{
+		{version: v1.Version, want: v1.SubtreeRootThreshold},
+		{version: v2.Version, want: v2.SubtreeRootThreshold},
+		{version: testground.Version, want: testground.SubtreeRootThreshold},
+	}
+	for _, tc := range testCases {
+		name := fmt.Sprintf("version %v", tc.version)
+		t.Run(name, func(t *testing.T) {
+			got := appconsts.SubtreeRootThreshold(tc.version)
+			assert.Equal(t, tc.want, got)
+		})
+	}
+}
+
+func TestSquareSizeUpperBound(t *testing.T) {
+	type testCase struct {
+		version uint64
+		want    int
+	}
+	testCases := []testCase{
+		{version: v1.Version, want: v1.SquareSizeUpperBound},
+		{version: v2.Version, want: v2.SquareSizeUpperBound},
+		{version: testground.Version, want: testground.SquareSizeUpperBound},
+	}
+	for _, tc := range testCases {
+		name := fmt.Sprintf("version %v", tc.version)
+		t.Run(name, func(t *testing.T) {
+			got := appconsts.SquareSizeUpperBound(tc.version)
+			assert.Equal(t, tc.want, got)
+		})
+	}
+}