tests/storage/stream_flash: Fix the test_stream_flash_erase_page

Fix for test that was failing due to incorrectly testing of
stream_flash_erase_page.
The stream_flash_erase_page would never be able to erase a page
it has been requested to erase.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>