test: drivers: flash: common: Copied size can't exceed page.size

flash_copy() is performed on a page.size span but following flash_read()
verification step is performed on EXPECTED_SIZE length (512).

This doesn't work on devices where page.size is lower than EXPECTED_SIZE,
such as STM32L1 where page size is 256.

To fix this, perform verification on the smalest value between page.size
and EXPECTED_SIZE.

Signed-off-by: Erwan Gouriou <erwan.gouriou@st.com>