support/testing: allow to use a multiplier for timeouts

Add a parameter to run-tests to act as a multiplier for all timeouts of
emulator.
It can be used to avoid sporadic failures on slow host machines as well
in elastic runners on the cloud.

Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Ricardo Martincoski <ricardo.martincoski@gmail.com>
[Arnout: rename multiplier to timeout_multiplier everywhere]
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>