tests: drivers: flash: size to erase equal to size to write

Need to erase as many bytes as the size to write.
The size to erase is expressed aligned on the page size.
(assuming the page_info.size is constant)
In case the page_info.size is lower than the size to write,
the next write operation will fail.

Signed-off-by: Francois Ramu <francois.ramu@st.com>