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/extent.c

@@ -1,9 +1,5 @@
 #include "test/jemalloc_test.h"
 
-#ifdef JEMALLOC_FILL
-const char *malloc_conf = "junk:false";
-#endif
-
 #include "test/extent_hooks.h"
 
 static void