zdtm: uptime_grow test

new testcase important for rebootless update testing:
we query system timer CLOCK_MONOTONIC continuously
and fail in case it stepped backwards

Signed-off-by: Evgeny Antyshev <eantyshev@parallels.com>
Acked-by: Andrew Vagin <avagin@parallels.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>