Add a test-case for small profiled allocations

Validate that small allocations (i.e. those with `size <= SC_SMALL_MAXCLASS`)
which are sampled for profiling maintain the expected invariants even
though they now take up less space.

test/unit/prof_small.sh

@@ -0,0 +1,6 @@
+#!/bin/sh
+
+if [ "x${enable_prof}" = "x1" ] ; then
+  export MALLOC_CONF="prof:true,lg_prof_sample:0"
+fi
+