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/zero.c

@@ -1,10 +1,5 @@
 #include "test/jemalloc_test.h"
 
-#ifdef JEMALLOC_FILL
-const char *malloc_conf =
-    "abort:false,junk:false,zero:true";
-#endif
-
 static void
 test_zero(size_t sz_min, size_t sz_max) {
 	uint8_t *s;