explain MALLOC_PERTURB_

Author: Christopher Zimmermann

test/dune

@@ -4,6 +4,7 @@
  (flags :standard -thread)
  (libraries alcotest bigstring bigstring-unix threads)
  (action
+ ; This enables runtime checks for use-after-free and double-free.
  (setenv MALLOC_CHECK_ 3
  (setenv MALLOC_PERTURB_ 195
  (run %{test} --color=always -q)))