tests: net: poll: Increase the poll timeout value

Got several sanity failures with this information

Assertion failed at tests/net/socket/poll/src/main.c:76: \
  test_poll: (tstamp >= 30U && tstamp <= 30 + FUZZ is false)

In one of the failed tests, the tstamp was 50. Dunno what is
wrong here but in order to proceed with this, increasing the
timeout temporarily. This commit can be removed if the root
cause for this timeout is figured out.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>