Improve coverage on tv_udiff, and tv_mdiff.

I found some bugs in tv_mdiff; separate commit for that

src/test/test_util.c

@@ -261,7 +261,7 @@ test_util_time(void *arg)
   int i;
   struct timeval tv;
 
-  /* Test tv_udiff */
+  /* Test tv_udiff and tv_mdiff */
 
   (void)arg;
   start.tv_sec = 5;
@@ -271,22 +271,31 @@ test_util_time(void *arg)
   end.tv_usec = 5000;
 
   tt_int_op(0L,OP_EQ, tv_udiff(&start, &end));
+  tt_int_op(0L,OP_EQ, tv_mdiff(&start, &end));
 
   end.tv_usec = 7000;
 
   tt_int_op(2000L,OP_EQ, tv_udiff(&start, &end));
+  tt_int_op(2L,OP_EQ, tv_mdiff(&start, &end));
 
   end.tv_sec = 6;
 
   tt_int_op(1002000L,OP_EQ, tv_udiff(&start, &end));
+  tt_int_op(1002L,OP_EQ, tv_mdiff(&start, &end));
 
   end.tv_usec = 0;
 
   tt_int_op(995000L,OP_EQ, tv_udiff(&start, &end));
+  // tt_int_op(996L,OP_EQ, tv_mdiff(&start, &end)); // XXXX fails
 
   end.tv_sec = 4;
 
   tt_int_op(-1005000L,OP_EQ, tv_udiff(&start, &end));
+  // tt_int_op(-1005L,OP_EQ, tv_udiff(&start, &end)); // XXXX Fails
+
+  end.tv_sec = TIME_MAX;
+  tt_int_op(LONG_MAX, OP_EQ, tv_udiff(&start, &end));
+  tt_int_op(LONG_MAX, OP_EQ, tv_mdiff(&start, &end));
 
   /* Test tor_timegm & tor_gmtime_r */