Add a very old SerializedFile

This file originates from Tests\EditModeAndPlayModeTests\NestedPrefabs\BackwardCompatibility\PrefabMergingTestScene.unity

UnityDataTool can read the header, example:

UnityDataTool serialized-file header v6format.unity
Version              6
Format               Legacy (32-bit)
File Size            18,079 bytes
Metadata Size        12,835 bytes
Data Offset          0
Endianness           Big Endian

However "dump" and "serialized-file objectlist" fails:
Fatal error. 0xC0000005

binary2text works to produce a text form (with recently added deprecation warning):

Serialized files [version 6] before 2019.1 are deprecated.
Unsupported type! Vector3f (error repeated several times)

Author: SkowronskiAndrew