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/prof_tctx.c

@@ -1,9 +1,5 @@
 #include "test/jemalloc_test.h"
 
-#ifdef JEMALLOC_PROF
-const char *malloc_conf = "prof:true,lg_prof_sample:0";
-#endif
-
 TEST_BEGIN(test_prof_realloc) {
 	tsdn_t *tsdn;
 	int flags;