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/unit/pack.sh

@@ -0,0 +1,4 @@
+#!/bin/sh
+
+# Immediately purge to minimize fragmentation.
+export MALLOC_CONF="decay_time:-1"