memory/function/mm_proactive_compaction: tweaks to mem-frag-test

In the change sets comprised in this work, we tweak mem-frag-test as follows:

• remove assumptions on THP size, decoupling the test from its original x86-centric design;
• adjust the testcase write_random_bytes() loop to reduce execution time;

Signed-off-by: Rafael Aquini raquini@redhat.com