small: fix flaky test in small

There was error in test: in case when rand() % OSCILLATION_MAX return 0,
no memory allocation is made, so fail_unless(obuf_capacity(&buf) > 0)
check failed. A small refactoring was also done: add slab_arena_destroy
for graceful resources release, removed global seed value, removed unused
value from enum.

Closes #5345

small @ 279fa38a

-Subproject commit 8ef2ec4b383e3b518fbfc462eee8d9c9786c3a64
+Subproject commit 279fa38ac89be526dcef0a9dd88cf4775367f5c5