Use MALLOC_CONF rather than malloc_conf for tests.

malloc_conf does not reliably work with MSVC, which complains of
"inconsistent dll linkage", i.e. its inability to support the
application overriding malloc_conf when dynamically linking/loading.
Work around this limitation by adding test harness support for per test
shell script sourcing, and converting all tests to use MALLOC_CONF
instead of malloc_conf.

test/integration/xallocx.sh

@@ -0,0 +1,5 @@
+#!/bin/sh
+
+if [ "x${enable_fill}" = "x1" ] ; then
+  export MALLOC_CONF="junk:false"
+fi