tests: Enable GNU libc memory allocation error checking

tests: Enable GNU libc memory allocation error checking

MALLOC_CHECK_=3 makes glibc check for memory allocation programming
errors.  It's the factory default, but set it anyway just in case
someone disabled it for speed.

Non-zero MALLOC_PERTURB_ makes glibc wipe memory value on allocation
and deallocation.  The actual value determines the bit pattern.  Set
it to the value of environment variable EMPIRE_CHECK_MALLOC_PERTURB or
else a pseudo-random number, and record it in sandbox/malloc-perturb.

See mallopt(3) for more information.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>