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_accum.c

@@ -5,11 +5,6 @@
 #define DUMP_INTERVAL		1
 #define BT_COUNT_CHECK_INTERVAL	5
 
-#ifdef JEMALLOC_PROF
-const char *malloc_conf =
-    "prof:true,prof_accum:true,prof_active:false,lg_prof_sample:0";
-#endif
-
 static int
 prof_dump_open_intercept(bool propagate_err, const char *filename) {
 	int fd;