sanitycheck: test on integration platforms on top of default platforms

When running sanitycheck without any platform filters (-p option), we
take the default platforms and run on those only. When the -G options is
used for integration testing, the platforms listed as integration
platforms are not being added on top of default platforms.

This change adds integration platforms to the list an treats
integration_platforms as a positive filter, so that
integration_platforms are always being considered.

Fixes #29829

Signed-off-by: Anas Nashif <anas.nashif@intel.com>