pool: add bunch of unit tests for NearlineStorageHandlerTest

Motivation:
better test coverage, 'documenting' expected/current behavior

Result:
better code coverage

Acked-by: Paul Millar
Target: master
Require-book: no
Require-notes: no
(cherry picked from commit 49b0ec3)
Signed-off-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>