test: add variable delta for poll.c test

For ZDTM purposes, poll.c uses delta of 5 seconds, which is fine, since
tests don't take a very long time to pass. However, in vzt-cpt, we start
a batch with multitude of tests and starting them all could take over an
hour. Hence, the test is bound to fail. An easy solution would be to
introduce an environment variable, which would allow to specify the
delta - ZDTM_DELTA. If it's unset (which is default behaviour in ZDTM) -
it would default to 5 seconds. However, if it's set (what we can do from
vzt-cpt) - then it will use delta which is set by our test.

travis-ci: success for test: add variable delta for poll.c test (rev2)
Signed-off-by: Vitaly Ostrosablin <vostrosablin@virtuozzo.com>
Reviewed-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>