test/subsys/storage/stream_flash: fix build size issue

This test uses SoC's flash from offset of 64 KB which might
overlap with executable for a target
(for instance nrf52840dk_nrf52840).
This patch moves this region to 128 KB which solves the issue.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>