add mockfile that holds a size instead of data

vfs_test.go

@@ -41,3 +41,22 @@ func TestVFS_Basic(t *testing.T) {
 		assert.NoError(t, err)
 	}
 }
+
+func TestVFS_MockFile(t *testing.T) {
+	vfs := NewVFS()
+	path := "test.txt"
+	sz := int64(8096)
+
+	err := vfs.CreateMockFile(path, sz, 0666)
+	assert.NoError(t, err)
+
+	fp, err := vfs.Open(path)
+	assert.NoError(t, err)
+
+	stat, err := fp.Stat()
+	assert.NoError(t, err)
+	assert.Equal(t, stat.Size(), sz)
+
+	err = fp.Close()
+	assert.NoError(t, err)
+}