tests: net: select: Increase the timeout to 20ms

The timeout might take more than 10ms in a heavily loaded system,
so increase the timeout to 20ms.

For example this is often seen for mps2_an385 platform.

  Assertion failed at \
  WEST_TOPDIR/zephyr/tests/net/socket/select/src/main.c:101: \
  test_select: (tstamp <= FUZZ is false)

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