twister: introduce twister-level timeout multiplier

Twister allows us to control maximum execution time for each test
with timeout value in test's .yaml configuration and
platform level timeout multiplier which allows us to tweak
timeout value for specific platform.

However, sometimes we want to additionally adjust tests timeouts
when running twister. This is especially useful in case of
simulation platform as simulation time may depend on the host
speed & load, we may select different (i.e. cycle accurate but
slower one) simulation method, etc...

Let's introduce global (twister-level) timeout multiplier option.

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