lib/store: Refactor common store code into helper function

A lot of tests use almost identical code for creating a batch buffer
which does a single write to memory and another is about to be added.
Instead, move the most generic version into a common helper function.
Unfortunately, the other instances are all subtly different enough to
make it not so trivial to try to use the helper. It could be done but
it is unclear if it is worth the effort at this point. This patch
proves the concept, if people like it enough then it can be extended.

v2: Fix up object address vs store offset confusion (with help from
Zbigniew K).
v3: Cope with >32bit store_offset (review feedback from Matthew Brost).

Signed-off-by: John Harrison <John.C.Harrison@Intel.com>
Reviewed-by: Matthew Brost <matthew.brost@intel.com>