boards: posix: native_posix: enable can_loopback0 by default

The .yaml file states that CAN is supported, but the basic sample
application samples/drivers/can/counter cannot be compiled without
additional configuration.

The loopback driver does not require any additional steps like the
linux SocketCAN driver, so it is safe to enable it by default and
get rid of the many overlay files in the tests.

ISO-TP tests and the counter sample are excluded via .yaml from
twister tests because of timing issues.

Signed-off-by: Martin Jäger <martin@libre.solar>