Remove #includes in tests

Since we're now including jemalloc_internal.h, all the required headers
are already pulled. This will avoid having to fiddle with headers that can
or can't be used with MSVC. Also, now that we use malloc_printf, we can use
util.h's definition of assert instead of assert.h's.

test/mremap.c

@@ -1,9 +1,3 @@
-#include <stdio.h>
-#include <stdlib.h>
-#include <assert.h>
-#include <errno.h>
-#include <string.h>
-
 #define	JEMALLOC_MANGLE
 #include "jemalloc_test.h"