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.c

@@ -1,8 +1,5 @@
 #include "test/jemalloc_test.h"
 
-/* Immediately purge to minimize fragmentation. */
-const char *malloc_conf = "decay_time:-1";
-
 /*
  * Size class that is a divisor of the page size, ideally 4+ regions per run.
  */