twister: implement platform-specific test timeout management

Twister allows us to control maximum execution time for each
test with timeout value in test .yaml configuration. This
helps us to prevent slow tests from stopping by timeout.

However it's hard to choose test timeout for all platforms
as some platforms are naturally slow. It could be a HW board with
power-efficient but slow CPU or simulation platform which
can perform instruction accurate simulation but does it slowly.

As we don't want to increase test timeout infinitely to meet
the needs of the slowest platform let's introduce
platform-specific test timeout management. It's implemented as
an optional 'timeout_multiplier' field in board .yaml
configuration. Setting it to some value multiplies each test
timeout by this value. By that we can increase timeouts only
for the platforms where it's required.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Evgeniy Paltsev <PaltsevEvgeniy@gmail.com>