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.