mtd-tests: use pages during readtest and stress

mtd-tests: use pages during readtest and stress

Same fix as a2c6bbc ("mtd-tests: Read and write pages during speed
tests") but applied to flash_readtest and flash_stress.

Resolves failure of flash_readtest when subpages are present. The test
reads a (sub)page followed by the entire OOB. Upon reaching the 2nd
subpage, the OOB read fails because it is beyond the end of OOB.

Signed-off-by: Ralph Siemsen <ralph.siemsen@linaro.org>
Signed-off-by: David Oberhollenzer <david.oberhollenzer@sigma-star.at>