twister: Fix DeviceHandler hangs without suitable DUT

Fixes an issue when DeviceHandler awaited for a suitable DUT infinitely.
It was possible if the device map has no devices with required platform,
or no devices with fixture needed for the test case, or devices without
serial connection. A trivial case is a typo in platform name.

Signed-off-by: Dmitrii Golovanov <dmitrii.golovanov@intel.com>