ut/blob: always use local blobstore pointer

Always call API on pointer that was received from blobstore
initialization or load, instead of relying on global one.

Where necessary added declaration, assignment and asserts.

Signed-off-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Change-Id: Ic4198ac984c13a8b77428deaa32ce9864e6750c1
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/942


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>