test: add test examples for prefixdb.go (#22752)

Co-authored-by: heren-ke <fghjtuy@163.com>

store/db/prefixdb_test.go

@@ -0,0 +1,49 @@
+package db_test
+
+import (
+	"testing"
+
+	"cosmossdk.io/store/db"
+	"cosmossdk.io/store/mock"
+	"github.com/stretchr/testify/require"
+	"go.uber.org/mock/gomock"
+)
+
+func TestPrefixDB(t *testing.T) {
+	mockCtrl := gomock.NewController(t)
+	defer mockCtrl.Finish()
+
+	mockDB := mock.NewMockKVStoreWithBatch(mockCtrl)
+	prefix := []byte("test:")
+	pdb := db.NewPrefixDB(mockDB, prefix)
+
+	key := []byte("key1")
+	value := []byte("value1")
+	mockDB.EXPECT().Set(gomock.Eq(append(prefix, key...)), gomock.Eq(value)).Return(nil)
+
+	err := pdb.Set(key, value)
+	require.NoError(t, err)
+
+	mockDB.EXPECT().Get(gomock.Eq(append(prefix, key...))).Return(value, nil)
+
+	returnedValue, err := pdb.Get(key)
+	require.NoError(t, err)
+	require.Equal(t, value, returnedValue)
+
+	mockDB.EXPECT().Has(gomock.Eq(append(prefix, key...))).Return(true, nil)
+
+	has, err := pdb.Has(key)
+	require.NoError(t, err)
+	require.True(t, has)
+
+	mockDB.EXPECT().Delete(gomock.Eq(append(prefix, key...))).Return(nil)
+
+	err = pdb.Delete(key)
+	require.NoError(t, err)
+
+	mockDB.EXPECT().Has(gomock.Eq(append(prefix, key...))).Return(false, nil)
+
+	has, err = pdb.Has(key)
+	require.NoError(t, err)
+	require.False(t, has)
+}