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/junk_alloc.c

@@ -1,3 +1 @@
-#define	JEMALLOC_TEST_JUNK_OPT "junk:alloc"
 #include "junk.c"
-#undef JEMALLOC_TEST_JUNK_OPT