g_usleep: round up the next millisecond on Windows. See #1371

The timer tests expect that a small value for sleep does not result in
no sleep at all. Round up to the next millisecond to bring it more in line
with other platforms.

This fixes the glib/timer tests.

This makes the 'threadtests' time out since that uses small usleeps a lot and
until now didn't wait at all, but now always waits a msec. Reduce the amount
of tests done on Windows to get the runtime down to something reasonable again.

https://bugzilla.gnome.org/show_bug.cgi?id=795569

glib/gtimer.c

@@ -251,7 +251,8 @@ void
 g_usleep (gulong microseconds)
 {
 #ifdef G_OS_WIN32
-  Sleep (microseconds / 1000);
+  /* Round up to the next millisecond */
+  Sleep (microseconds ? (1 + (microseconds - 1) / 1000) : 0);
 #else
   struct timespec request, remaining;
   request.tv_sec = microseconds / G_USEC_PER_SEC;