conf: another refactor of virstoragetest
    
    Another reduction in the number of structs I have to modify
    when I start tracking new fields in virStorageFileMetadata.
    
    * tests/virstoragetest.c (_testFileData): Add fields.
    (testStorageChain): Select between fields based on flag.
    (mymain): Record both absolute and relative expectations in one
    struct.
    
    Signed-off-by: Eric Blake <eblake@redhat.com>