Before this patch, tst-readdir.cc assert()ed on every test, meaning that
a failure will cause a crash. Change this to use a report() function,
which counts failures instead of immediately crashing on the first one.

This patch doesn't change anything in what this test actually tests for.

Signed-off-by: Nadav Har'El <nyh@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>