tests: Support OT_SKIP_READDIR_RAND
tests: Support OT_SKIP_READDIR_RAND

For some reason I'm really not inclined to debug right now, `libtool`
is eating the `-fsanitize=address` argument when linking
`libreaddir-rand.so`, which causes failures since we're trying to
`LD_PRELOAD`.

Closes: #351
Approved by: jlebon

GNOME/ostree,gatispaeglis/ostree,GNOME/ostree,gatispaeglis/ostree,GNOME/ostree,GNOME/ostree,gatispaeglis/ostree,gatispaeglis/ostree,gatispaeglis/ostree,gatispaeglis/ostree,GNOME/ostree