tests: use g_usleep instead of rem = sleep(time)

tests: use g_usleep instead of rem = sleep(time)

Relying on sleep to always return having slept isn't safe as a signal
may have occurred. If signals are constantly incoming the program will
never reach its termination condition. This is believed to be the
mechanism causing time outs for qht-test in Travis.

The glib g_usleep() deals with all of this for us so lets use it instead.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>