tests/drivers uart_mix_fifo_poll: Allow configuring test length

This test is quite heavy and long in simulation,
but its length is meant to find unlikely issues which
may be triggered only very rarely.
Let's provide a kconfig value to chose how long the
test is, and set it to a lower value when running in simulation
(in CI) to save time.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>