Merge branch 'ep/malloc-check-perturb'

Run our test scripts with MALLOC_CHECK_ and MALLOC_PERTURB_, the
built-in memory access checking facility GNU libc has.

* ep/malloc-check-perturb:
  MALLOC_CHECK: various clean-ups
  Add MALLOC_CHECK_ and MALLOC_PERTURB_ libc env to the test suite for detecting heap corruption