hdf5/cmem: TestEncode restores the wrong endianness value

package cmem test fails with master:

cmem/encoder_test.go:43:4: oldEndian declared but not used

that's because:

oldEndian := nativeEndian
nativeEndian = binary.LittleEndian
defer func() { oldEndian = nativeEndian }()

it restores the wrong endianness :)