kunit: add is_init test attribute

Add is_init test attribute of type bool. Add to_string, get, and filter
methods to lib/kunit/attributes.c.

Mark each of the tests in the init section with the is_init=true attribute.

Add is_init to the attributes documentation.

Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Rae Moar <rmoar@google.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>