lib1506: make sure the transfers are not within the same ms

Just to make sure the test is properly repeatable.

Bug: http://curl.haxx.se/mail/lib-2014-05/0081.html
Reported-by: Henrik

tests/libtest/lib1506.c

@@ -114,6 +114,7 @@ int test(char *URL)
 
       abort_on_test_timeout();
     }
+    wait_ms(1); /* to ensure different end times */
   }
 
 test_cleanup: