selftests: vm: add KSM huge pages merging time test

Add test case of KSM merging time using mostly huge pages

Link: https://lkml.kernel.org/r/20211013044045.360251-1-pedrodemargomes@gmail.com
Signed-off-by: Pedro Demarchi Gomes <pedrodemargomes@gmail.com>
Cc: Zhansaya Bagdauletkyzy <zhansayabagdaulet@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>