twister: Include coverage support for device testing

Include coverage support for device testing. It works by switching
capture_coverage to True (as it would meet coverage data start
pattern). Then we continue tot read coverage data until we get
coverage data end pattern. Otherwise, after receiving test result
pattern, we close serial console and do not get time to find start
coverage data pattern.

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>