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: NEric Blake <eblake@redhat.com>