make `--dist loadgroup` the default value

[jnhyperion]

As the doc said:

This will make sure test1 and TestA::test2 will run in the same worker. Tests without the xdist_group mark are distributed normally as in the --dist=load mode.

the --dist loadgroup could be just a kind of extension of --dist load, make it as default value should be safe and also very convenient to users too.