valgrind: print failed ptest details

Some intermittent failures in valgrind are hard
reproduce.

Printing the difference between actual and expected
will make understanding them slightly easier.

[YOCTO #14294]

Signed-off-by: Yi Fan Yu <yifan.yu@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>