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

tests/libtest.sh

@@ -80,7 +80,14 @@ fi
 if test -n "${OT_TESTS_VALGRIND:-}"; then
     CMD_PREFIX="env G_SLICE=always-malloc OSTREE_SUPPRESS_SYNCFS=1 valgrind -q --error-exitcode=1 --leak-check=full --num-callers=30 --suppressions=${test_srcdir}/glib.supp --suppressions=${test_srcdir}/ostree.supp"
 else
-    CMD_PREFIX="env LD_PRELOAD=${test_builddir}/libreaddir-rand.so"
+    # In some cases the LD_PRELOAD may cause obscure problems,
+    # e.g. right now it breaks for me with -fsanitize=address, so
+    # let's allow users to skip it.
+    if test -z "${OT_SKIP_READDIR_RAND:-}"; then
+	CMD_PREFIX="env LD_PRELOAD=${test_builddir}/libreaddir-rand.so"
+    else
+	CMD_PREFIX=""
+    fi
 fi
 
 assert_streq () {