kunit: kunit.py extract handlers

The main function contains a wide if-elif block that handles different
subcommands. It's possible to make code refactoring to extract
subcommands handlers.

Fixed commit summary line.
Shuah Khan <skhan@linuxfoundation.org>

Signed-off-by: Alexander Pantyukhin <apantykhin@gmail.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>