bigarray/tests: Factor out a way to spcify on which BigFile/BigFileH an array is tested into fixture parameter

Currently we have only one BigFile and its BigFileH handle. However in
the next patch, for RAMArray, we'll be adding handles for opened RAM
files, and it would be good to test whole BigArray functionality on
data served by those handles too.

Prepare for this and first factor out into testbig fixture the way to
open such handles.